diff --git a/08-concurrency-foundations/60-contexts/flight/flight.go b/08-concurrency-foundations/60-contexts/flight/flight.go
new file mode 100644
index 0000000..0f121de
--- /dev/null
+++ b/08-concurrency-foundations/60-contexts/flight/flight.go
@@ -0,0 +1,6 @@
+package flight
+
+type Position struct {
+	Lat float32
+	Lng float32
+}
diff --git a/08-concurrency-foundations/60-contexts/main.go b/08-concurrency-foundations/60-contexts/main.go
index 15caf93..5289bab 100644
--- a/08-concurrency-foundations/60-contexts/main.go
+++ b/08-concurrency-foundations/60-contexts/main.go
@@ -3,8 +3,25 @@ package main
 import (
 	"context"
 	"net/http"
+	"time"
+
+	"github.com/teivah/100-go-mistakes/08-concurrency-foundations/60-contexts/flight"
 )
 
+type publisher interface {
+	Publish(ctx context.Context, position flight.Position) error
+}
+
+type publishHandler struct {
+	pub publisher
+}
+
+func (h publishHandler) publishPosition(position flight.Position) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 4*time.Second)
+	defer cancel()
+	return h.pub.Publish(ctx, position)
+}
+
 type key string
 
 const isValidHostKey key = "isValidHost"
diff --git a/09-concurrency-practice/63-goroutines-loop-variables/main.go b/09-concurrency-practice/63-goroutines-loop-variables/main.go
index bdfe51d..7f06c18 100644
--- a/09-concurrency-practice/63-goroutines-loop-variables/main.go
+++ b/09-concurrency-practice/63-goroutines-loop-variables/main.go
@@ -27,8 +27,8 @@ func listing3() {
 	s := []int{1, 2, 3}
 
 	for _, i := range s {
-		go func(i int) {
-			fmt.Print(i)
+		go func(val int) {
+			fmt.Print(val)
 		}(i)
 	}
 }
diff --git a/09-concurrency-practice/70-mutex-slices-maps/main.go b/09-concurrency-practice/70-mutex-slices-maps/main.go
index 662abdb..23b8b48 100644
--- a/09-concurrency-practice/70-mutex-slices-maps/main.go
+++ b/09-concurrency-practice/70-mutex-slices-maps/main.go
@@ -17,7 +17,7 @@ func main() {
 }
 
 type Cache struct {
-	mu       sync.Mutex
+	mu       sync.RWMutex
 	balances map[string]float64
 }
 
@@ -28,9 +28,9 @@ func (c *Cache) AddBalance(id string, balance float64) {
 }
 
 func (c *Cache) AverageBalance1() float64 {
-	c.mu.Lock()
+	c.mu.RLock()
 	balances := c.balances
-	c.mu.Unlock()
+	c.mu.RUnlock()
 
 	sum := 0.
 	for _, balance := range balances {
@@ -40,8 +40,8 @@ func (c *Cache) AverageBalance1() float64 {
 }
 
 func (c *Cache) AverageBalance2() float64 {
-	c.mu.Lock()
-	defer c.mu.Unlock()
+	c.mu.RLock()
+	defer c.mu.RUnlock()
 
 	sum := 0.
 	for _, balance := range c.balances {
@@ -51,12 +51,12 @@ func (c *Cache) AverageBalance2() float64 {
 }
 
 func (c *Cache) AverageBalance3() float64 {
-	c.mu.Lock()
+	c.mu.RLock()
 	m := make(map[string]float64, len(c.balances))
 	for k, v := range c.balances {
 		m[k] = v
 	}
-	c.mu.Unlock()
+	c.mu.RUnlock()
 
 	sum := 0.
 	for _, balance := range m {
diff --git a/10-standard-lib/78-sql/rows-iterations-errors/main.go b/10-standard-lib/78-sql/rows-iterations-errors/main.go
index 9e3e70c..8a1334a 100644
--- a/10-standard-lib/78-sql/rows-iterations-errors/main.go
+++ b/10-standard-lib/78-sql/rows-iterations-errors/main.go
@@ -6,8 +6,8 @@ import (
 	"log"
 )
 
-func (c *conn) get1(ctx context.Context, id string) (string, int, error) {
-	rows, err := c.db.QueryContext(ctx,
+func get1(ctx context.Context, db *sql.DB, id string) (string, int, error) {
+	rows, err := db.QueryContext(ctx,
 		"SELECT DEP, AGE FROM EMP WHERE ID = ?", id)
 	if err != nil {
 		return "", 0, err
@@ -33,8 +33,8 @@ func (c *conn) get1(ctx context.Context, id string) (string, int, error) {
 	return department, age, nil
 }
 
-func (c *conn) get2(ctx context.Context, id string) (string, int, error) {
-	rows, err := c.db.QueryContext(ctx,
+func get2(ctx context.Context, db *sql.DB, id string) (string, int, error) {
+	rows, err := db.QueryContext(ctx,
 		"SELECT DEP, AGE FROM EMP WHERE ID = ?", id)
 	if err != nil {
 		return "", 0, err
@@ -62,7 +62,3 @@ func (c *conn) get2(ctx context.Context, id string) (string, int, error) {
 
 	return department, age, nil
 }
-
-type conn struct {
-	db *sql.DB
-}
diff --git a/11-testing/87-time-api/listing1/main.go b/11-testing/87-time-api/listing1/main.go
index 8ee2c2e..5b483ec 100644
--- a/11-testing/87-time-api/listing1/main.go
+++ b/11-testing/87-time-api/listing1/main.go
@@ -15,7 +15,7 @@ type Event struct {
 	Data      string
 }
 
-func (c *Cache) TrimBefore(since time.Duration) {
+func (c *Cache) TrimOlderThan(since time.Duration) {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
 
diff --git a/11-testing/87-time-api/listing1/main_test.go b/11-testing/87-time-api/listing1/main_test.go
index 089c4f3..8c02799 100644
--- a/11-testing/87-time-api/listing1/main_test.go
+++ b/11-testing/87-time-api/listing1/main_test.go
@@ -5,7 +5,7 @@ import (
 	"time"
 )
 
-func TestCache_TrimBefore(t *testing.T) {
+func TestCache_TrimOlderThan(t *testing.T) {
 	events := []Event{
 		{Timestamp: time.Now().Add(-20 * time.Millisecond)},
 		{Timestamp: time.Now().Add(-10 * time.Millisecond)},
@@ -13,7 +13,7 @@ func TestCache_TrimBefore(t *testing.T) {
 	}
 	cache := &Cache{}
 	cache.Add(events)
-	cache.TrimBefore(15 * time.Millisecond)
+	cache.TrimOlderThan(15 * time.Millisecond)
 	got := cache.GetAll()
 	expected := 2
 	if len(got) != expected {
diff --git a/11-testing/87-time-api/listing2/main.go b/11-testing/87-time-api/listing2/main.go
index d06c88e..6d0d944 100644
--- a/11-testing/87-time-api/listing2/main.go
+++ b/11-testing/87-time-api/listing2/main.go
@@ -25,7 +25,7 @@ type Event struct {
 	Data      string
 }
 
-func (c *Cache) TrimBefore(since time.Duration) {
+func (c *Cache) TrimOlderThan(since time.Duration) {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
 
diff --git a/11-testing/87-time-api/listing2/main_test.go b/11-testing/87-time-api/listing2/main_test.go
index ae181c4..f977ee0 100644
--- a/11-testing/87-time-api/listing2/main_test.go
+++ b/11-testing/87-time-api/listing2/main_test.go
@@ -5,7 +5,7 @@ import (
 	"time"
 )
 
-func TestCache_TrimBefore(t *testing.T) {
+func TestCache_TrimOlderThan(t *testing.T) {
 	events := []Event{
 		{Timestamp: parseTime(t, "2020-01-01T12:00:00.04Z")},
 		{Timestamp: parseTime(t, "2020-01-01T12:00:00.05Z")},
@@ -15,7 +15,7 @@ func TestCache_TrimBefore(t *testing.T) {
 		return parseTime(t, "2020-01-01T12:00:00.06Z")
 	}}
 	cache.Add(events)
-	cache.TrimBefore(15 * time.Millisecond)
+	cache.TrimOlderThan(15 * time.Millisecond)
 	// ...
 }
 
diff --git a/11-testing/87-time-api/listing3/main.go b/11-testing/87-time-api/listing3/main.go
index fed414e..89f09e8 100644
--- a/11-testing/87-time-api/listing3/main.go
+++ b/11-testing/87-time-api/listing3/main.go
@@ -15,7 +15,7 @@ type Event struct {
 	Data      string
 }
 
-func (c *Cache) TrimBefore(now time.Time, since time.Duration) {
+func (c *Cache) TrimOlderThan(now time.Time, since time.Duration) {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
 
diff --git a/11-testing/87-time-api/listing3/main_test.go b/11-testing/87-time-api/listing3/main_test.go
index 501cd82..762a008 100644
--- a/11-testing/87-time-api/listing3/main_test.go
+++ b/11-testing/87-time-api/listing3/main_test.go
@@ -5,7 +5,7 @@ import (
 	"time"
 )
 
-func TestCache_TrimBefore(t *testing.T) {
+func TestCache_TrimOlderThan(t *testing.T) {
 	events := []Event{
 		{Timestamp: parseTime(t, "2020-01-01T12:00:00.04Z")},
 		{Timestamp: parseTime(t, "2020-01-01T12:00:00.05Z")},
@@ -13,7 +13,7 @@ func TestCache_TrimBefore(t *testing.T) {
 	}
 	cache := &Cache{}
 	cache.Add(events)
-	cache.TrimBefore(parseTime(t, "2020-01-01T12:00:00.06Z"), 15*time.Millisecond)
+	cache.TrimOlderThan(parseTime(t, "2020-01-01T12:00:00.06Z"), 15*time.Millisecond)
 	got := cache.GetAll()
 	expected := 2
 	if len(got) != expected {
diff --git a/11-testing/87-time-api/listing4/main.go b/11-testing/87-time-api/listing4/main.go
new file mode 100644
index 0000000..5b6ac00
--- /dev/null
+++ b/11-testing/87-time-api/listing4/main.go
@@ -0,0 +1,42 @@
+package listing1
+
+import (
+	"sync"
+	"time"
+)
+
+type Cache struct {
+	mu     sync.RWMutex
+	events []Event
+}
+
+type Event struct {
+	Timestamp time.Time
+	Data      string
+}
+
+func (c *Cache) TrimOlderThan(t time.Time) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	for i := 0; i < len(c.events); i++ {
+		if c.events[i].Timestamp.After(t) {
+			c.events = c.events[i:]
+			return
+		}
+	}
+}
+
+func (c *Cache) Add(events []Event) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.events = append(c.events, events...)
+}
+
+func (c *Cache) GetAll() []Event {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	return c.events
+}
diff --git a/11-testing/87-time-api/listing4/main_test.go b/11-testing/87-time-api/listing4/main_test.go
new file mode 100644
index 0000000..68f0f38
--- /dev/null
+++ b/11-testing/87-time-api/listing4/main_test.go
@@ -0,0 +1,31 @@
+package listing1
+
+import (
+	"testing"
+	"time"
+)
+
+func TestCache_TrimOlderThan(t *testing.T) {
+	events := []Event{
+		{Timestamp: parseTime(t, "2020-01-01T12:00:00.04Z")},
+		{Timestamp: parseTime(t, "2020-01-01T12:00:00.05Z")},
+		{Timestamp: parseTime(t, "2020-01-01T12:00:00.06Z")},
+	}
+	cache := &Cache{}
+	cache.Add(events)
+	cache.TrimOlderThan(parseTime(t, "2020-01-01T12:00:00.06Z").
+		Add(-15 * time.Millisecond))
+	got := cache.GetAll()
+	expected := 2
+	if len(got) != expected {
+		t.Fatalf("expected %d, got %d", expected, len(got))
+	}
+}
+
+func parseTime(t *testing.T, timestamp string) time.Time {
+	ts, err := time.Parse(time.RFC3339, timestamp)
+	if err != nil {
+		t.FailNow()
+	}
+	return ts
+}
diff --git a/11-testing/88-utility-package/iotest/main.go b/11-testing/88-utility-package/iotest/main.go
index bfb8593..ec37f84 100644
--- a/11-testing/88-utility-package/iotest/main.go
+++ b/11-testing/88-utility-package/iotest/main.go
@@ -12,7 +12,7 @@ func (l LowerCaseReader) Read(p []byte) (int, error) {
 	return 0, nil
 }
 
-func foo(r io.Reader) error {
+func foo1(r io.Reader) error {
 	b, err := io.ReadAll(r)
 	if err != nil {
 		return err
@@ -23,6 +23,17 @@ func foo(r io.Reader) error {
 	return nil
 }
 
+func foo2(r io.Reader) error {
+	b, err := readAll(r, 3)
+	if err != nil {
+		return err
+	}
+
+	// ...
+	_ = b
+	return nil
+}
+
 func readAll(r io.Reader, retries int) ([]byte, error) {
 	b := make([]byte, 0, 512)
 	for {
@@ -33,10 +44,10 @@ func readAll(r io.Reader, retries int) ([]byte, error) {
 		b = b[:len(b)+n]
 		if err != nil {
 			if err == io.EOF {
-				err = nil
+				return b, nil
 			}
 			retries--
-			if retries <= 0 {
+			if retries < 0 {
 				return b, err
 			}
 		}
diff --git a/11-testing/88-utility-package/iotest/main_test.go b/11-testing/88-utility-package/iotest/main_test.go
index fc127eb..16ba851 100644
--- a/11-testing/88-utility-package/iotest/main_test.go
+++ b/11-testing/88-utility-package/iotest/main_test.go
@@ -16,8 +16,17 @@ func TestLowerCaseReader(t *testing.T) {
 	}
 }
 
-func TestFoo(t *testing.T) {
-	err := foo(iotest.TimeoutReader(
+func TestFoo1(t *testing.T) {
+	err := foo1(iotest.TimeoutReader(
+		strings.NewReader(randomString(1024)),
+	))
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestFoo2(t *testing.T) {
+	err := foo2(iotest.TimeoutReader(
 		strings.NewReader(randomString(1024)),
 	))
 	if err != nil {
diff --git a/11-testing/90-testing-features/setup-teardown/main_test.go b/11-testing/90-testing-features/setup-teardown/main_test.go
index 41120e2..ed659a1 100644
--- a/11-testing/90-testing-features/setup-teardown/main_test.go
+++ b/11-testing/90-testing-features/setup-teardown/main_test.go
@@ -7,8 +7,8 @@ import (
 )
 
 func TestMySQLIntegration(t *testing.T) {
-	s := setupMySQL()
-	defer teardownMySQL(s)
+	setupMySQL()
+	defer teardownMySQL()
 
 	// ...
 }
@@ -26,12 +26,12 @@ func createConnection(t *testing.T, dsn string) *sql.DB {
 }
 
 func TestMain(m *testing.M) {
-	s := setupMySQL()
+	setupMySQL()
 	code := m.Run()
-	teardownMySQL(s)
+	teardownMySQL()
 	os.Exit(code)
 }
 
-func setupMySQL() interface{} { return nil }
+func setupMySQL() {}
 
-func teardownMySQL(m interface{}) {}
+func teardownMySQL() {}
diff --git a/11-testing/90-testing-features/utility-function/main.go b/11-testing/90-testing-features/utility-function/main.go
index 15a501b..38423f0 100644
--- a/11-testing/90-testing-features/utility-function/main.go
+++ b/11-testing/90-testing-features/utility-function/main.go
@@ -1,22 +1,5 @@
 package main
 
-func NewStore() Store {
-	return Store{}
-}
-
-type Store struct{}
-
-func (s Store) PutCustomer(Customer) error {
-	return nil
-}
-
 type Customer struct {
 	id string
 }
-
-func customerFactory(id string) (Customer, error) {
-	if id == "" {
-		return Customer{}, nil
-	}
-	return Customer{id: id}, nil
-}
diff --git a/11-testing/90-testing-features/utility-function/main_test.go b/11-testing/90-testing-features/utility-function/main_test.go
index eaee7ec..f09e3c9 100644
--- a/11-testing/90-testing-features/utility-function/main_test.go
+++ b/11-testing/90-testing-features/utility-function/main_test.go
@@ -1,36 +1,44 @@
 package main
 
-import "testing"
+import (
+	"errors"
+	"testing"
+)
 
-func TestPutCustomer1(t *testing.T) {
-	customer, err := createCustomer1()
+func TestCustomer1(t *testing.T) {
+	customer, err := createCustomer1("foo")
 	if err != nil {
 		t.Fatal(err)
 	}
-	err = NewStore().PutCustomer(customer)
-	// Check err
-	_ = err
+	// ...
+	_ = customer
 }
 
-func createCustomer1() (Customer, error) {
-	customer, err := customerFactory("foo")
+func createCustomer1(someArg string) (Customer, error) {
+	customer, err := customerFactory(someArg)
 	if err != nil {
-		return Customer{}, nil
+		return Customer{}, err
 	}
 	return customer, nil
 }
 
-func TestPutCustomer(t *testing.T) {
-	customer := createCustomer2(t)
-	err := NewStore().PutCustomer(customer)
-	// Check err
-	_ = err
+func TestCustomer2(t *testing.T) {
+	customer := createCustomer2(t, "foo")
+	// ...
+	_ = customer
 }
 
-func createCustomer2(t *testing.T) Customer {
-	customer, err := customerFactory("foo")
+func createCustomer2(t *testing.T, someArg string) Customer {
+	customer, err := customerFactory(someArg)
 	if err != nil {
 		t.Fatal(err)
 	}
 	return customer
 }
+
+func customerFactory(someArg string) (Customer, error) {
+	if someArg == "" {
+		return Customer{}, errors.New("empty")
+	}
+	return Customer{id: someArg}, nil
+}