JuniorTheoryVery common

What does the program print?

package main

import "fmt"

func main() {
    s := []int{1, 2}
    t := append(s, 3)
    t[0] = 9
    fmt.Println(s[0], len(s), len(t))
}

1 2 3

Expecting 9 2 3: s has no free capacity, so append copies the elements into a new array, and t[0] = 9 changes only that copy.
Expecting 1 3 3 or an error: append never changes s itself, and a full slice is not an error, it just moves to a bigger array.

JuniorTheoryVery common

package main

import "fmt"

func main() {
    s := []int{1, 2}
    t := append(s, 3)
    t[0] = 9
    fmt.Println(s[0], len(s), len(t))
}

1 2 3

Expecting 9 2 3: s has no free capacity, so append copies the elements into a new array, and t[0] = 9 changes only that copy.
Expecting 1 3 3 or an error: append never changes s itself, and a full slice is not an error, it just moves to a bigger array.