Just tell him that you knew a guy who went by "Bhobb" and since then you always make sure to ask.

You should have said sweetly your last name please:-)

Whenever something this backfires i usually say something along the lines of "well some people named bob go by bobby or rob sometimes even bill i wasn't quite sure" or you can go with the classic "how do you want your name in my phone?"

Didn't we have an FML exactly like this one, except the name is Rob?

Nice..

Perhaps it's not even Bob - and he just wanted to see how little attention you spent...

Am I the only one who thought of Katt Williams Bkob skit here?

I don't blame you. Formal introductions are too rare these days. When I meet people in a group setting, it's easy for the new names to slide by me in the hubbub. I just keep learning to pay more attention to these things since it's nearly landed me in awkward messes, also.

That's when you just say "I meant your last name" and hope it isn't Smith..

This has happened to me, except with the name "Will". Beyond embarrassing.