by byron.rakitzis:

Go version 1.3 (and 1.2)

The documentation states that crypto/rand.Read() returns a byte count equal to the
length of the incoming slice whenever err==nil. This is contradicted with our code and
is reproducible on the playground:

http://play.golang.org/p/xd3W7l4Q-l

Source:

package main

import (
    "crypto/rand"
    "fmt"
)

func foo(i int) error {
    buf := make([]byte, 20)
    n, err := rand.Reader.Read(buf)
    switch {
    case err != nil:
        return fmt.Errorf("(%dth iteration) Rand reader error: %s", i, err)
    case n != len(buf):
        // err is nil!!!
        return fmt.Errorf("(%dth iteration) didn't get the bytes we requested (%d), got %d", i, len(buf), n)
    default:
        return nil
    }
}

func main() {
    for i := 0; i < 1000000; i++ {
        if err := foo(i); err != nil {
            panic(err)
        }
    }
}