Is this normal? I'm 32 years old. I've probably been to hundreds of parties/bbq's/birthdays/etc, and I've never been asked for cash ex post facto.
A few facts:
1. We've been co-workers for over a year. I know him decently well, as we've gone to bars and restaurants with friends before
2. I did not vomit
3. I did not break anything
4. I asked ahead: "can I give you some cash?", and "can I bring any food or alcohol?" In both cases he said "don't worry" and "it's taken care of"
5. Other people also got charged $50
Sounds like your friend is a bit clueless on this one. General rule is when you invite someone over to your house, you are the host and shouldn’t expect guests to pay. If the expectation is otherwise, letting guests know before hand – i.e. this is a potluck, or let’s split the costs on this event is usually good form.
It seems as though your friend sees himself as event organizer and not a host. For example, “I have baseball tickets, I want to invite you to a baseball game” the inviter is clearly a host and the inviter pays. On the other hand “Let's go catch a baseball game with our group of friends” the asker is an event organizer and the organizer is fine expecting everyone to pay for themselves. At a restaurant this might be slightly different depending on your social circle’s norms. I.e. do you go out weekly and take turns paying or do you go dutch?
I would indicate to your friend your surprise at being charged after the fact and that there seems to be a misunderstanding. If your friend did intend to host, then point out to him the host usually pays. If your friend insists on you paying, I would agree to pay, but hold off paying until he requests payment again. This gives him the opportunity to quietly drop his payment request.
Good friends don’t come by easily. Each of our friends we like for different reasons. Not all of our friends have to be well mannered, generous, funny, intelligent, good with people etc. $50 to smooth things out with a friend who sounds like he needs the money might be a small price of this friendship.
Finally in the future, avoid invitations with this friend. Only go on outings where everyone can easily pay their own way.