What happens if you send on a closed channel inside a select with a default?

ch := make(chan int, 1)
close(ch)
select {
case ch <- 1:
	fmt.Println("sent")
default:
	fmt.Println("default")
}

It panics with send on closed channel. It does not fall through to default. A send case on a closed channel counts as "ready" (it can proceed immediately), and proceeding means panicking. select gives you no protection here.

A related trap: people sometimes try to write a "safe send" like this:

func trySend(ch chan int, v int) (ok bool) {
	defer func() {
		if recover() != nil { ok = false }
	}()
	ch <- v
	return true
}

It works, but it's a code smell that signals broken ownership. The right fix is to make sure only the sender side closes, after all sends are done, or to have senders select on a separate done channel instead of ever closing the data channel. Interviewers ask this to see whether you know that closed-channel semantics differ between send (panic) and receive (zero value).