Common mistakes: Add "using reference to loop iterator variable"

CommonMistakes.md

@@ -2,12 +2,72 @@ Table of Contents
 =================
 
 + [Introduction](#introduction)
++ [Using reference to loop iterator variable](#using-reference-to-loop-iterator-variable)
 + [Using goroutines on loop iterator variables](#using-goroutines-on-loop-iterator-variables)
 
 # Introduction
 
 When new programmers start using Go or when old Go programmers start using a new concept, there are some common mistakes that many of them make.  Here is a non-exhaustive list of some frequent mistakes that show up on the mailing lists and in IRC.
 
+# Using reference to loop iterator variable
+
+In Go, the loop iterator variable is a single variable that takes different values in each loop iteration. This is very efficient, but might lead to unintended behavior when used incorrectly. For example, see the following program:
+
+```go
+func main() {
+	var out []*int
+	for i := 0; i < 3; i++ {
+		out = append(out, &i)
+	}
+	fmt.Println("Values:", *out[0], *out[1], *out[2])
+	fmt.Println("Addresses:", out[0], out[1], out[2])
+}
+```
+
+It will output unexpected results:
+
+```
+Values: 3 3 3
+Addresses: 0x40e020 0x40e020 0x40e020
+```
+
+Explanation: in each iteration we append the address of `i` to the `out` slice, but since it is the same variable, we append the same address which eventually contains the last value that was assigned to `i`. One of the solutions is to copy the loop variable into a new variable:
+
+```diff
+ for i := 0; i < 3; i++ {
++	i := i // Copy i into a new variable.
+ 	out = append(out, &i)
+ }
+```
+
+The new output of the program is what was expected:
+
+```
+Values: 0 1 2
+Addresses: 0x40e020 0x40e024 0x40e028
+```
+
+Explanation: the line `i := i` copies the loop variable `i` into a new variable scoped to the for loop body block, also called `i`. The address of the new variable is the one that is appended to the array, which makes it outlive the for loop body block. In each loop iteration a new variable is created.
+
+While this example might look a bit obvious, the same unexpected behavior could be more hidden in some other cases. For example, the loop variable can be an array and the reference can be a slice:
+
+```go
+func main() {
+	var out [][]int
+	for _, i := range [][1]int{{1}, {2}, {3}} {
+		out = append(out, i[:])
+	}
+	fmt.Println("Values:", out)
+}
+```
+
+Output:
+```
+Values: [[3] [3] [3]]
+```
+
+The same issue can be demonstrated also when the loop variable is being used in a Goroutine (see the following section).
+
 # Using goroutines on loop iterator variables 
 
 When iterating in Go, one might attempt to use goroutines to process data in parallel. For example, you might write something like this, using a closure: