[Cyber Liberty]

On many (but not all) firearms, it would then be possible to remove the magazine, insert another round, and re-insert the magazine.

I can see how such a situation could cause the malfunction we saw here. Zimmerman's gun (Kel-Tec?), reportedly, did not chamber the second round. If he crammed it to get the extra round, the auto-feed may not have worked properly, thus giving him only one shot. Let that be a lesson to everybody: You don't need the one extra round that badly!

[MrEdd]

I suspect it didn’t chamber because Trayvon was holding the slide.