In Go, the append function reallocates the underlying array if there's no more room. Hence, if you have two slices that point to the same array, after you append to one of the slices, the other slice may or may not point to the same array. This can lead to some unexpected behavior:
package main
import "fmt"
func main() {
// Create a slice of length 1 and capacity 2.
a := make([]int, 1, 2)
// b "aliases" a
b := a
// If I set something in a, you see it in b.
a[0] = 1
fmt.Println(a[0], b[0]) // Prints 1 1; they're equal.
// append returns a new slice, but with the same array.
// Hence, if I set something in a, you still see it in b.
a = append(a, 2)
a[0] = 2
fmt.Println(a[0], b[0]) // Prints 2 2; they're equal.
// I'm doing the same thing as last time. However, this time,
// append has to allocate a new array because there's not enough
// space. Hence, if I set something in a, you don't see it in b.
a = append(a, 3)
a[0] …