I work retail - thankfully in Canada, where Black Friday is not as bad - and I have one thing I'd like to suggest to make my fellow workers a little less miserable.

If something goes wrong - you can't get your item, the price is wrong, etc - remember it's due to the store, not the person you're dealing with.

That said, if you do flip out and you feel bad, apologize if you like but don't say "I know that it's not your fault and you can't do anything about it." I can't speak for other retail people, but it's actually more frustrating when people say that AFTER freaking out on you.