jeffb,
Your: ".... once they had a 500 pound bomb go right through the wingtip, dropped from an unseen ac above them. Luckily, it neither exploded, nor hit anything vital".
There is a bit of newsreel footage which has been shown on air hundreds of times since the war. It shows a bomb-release film over a burning German city: clearly silhouetted against the fires is another Lancaster some distance below, but exactly underneath the aircraft that has just bombed.
With the densely packed bomber streams, it must have happened many times. If the lower aircraft were destroyed this way, no one would be any the wiser.
Danny.