When someone sends you an evite to a party and you have to create an account to reply.
Followed closely in the Irksome Olympics by that same rsvp requiring that you not only tell them what category of food you'll be bringing (and oh yes, it's a potluck, which I did not know until RIGHT NOW), but exactly what dish you'll be bringing. I clicked "dessert" and typed in "Something" as my answer.
Argh. First off, don't use a software program that requires attendees to set up accounts. Secondly, tell attendees the party is a potluck from the outset. Third, do not insist on trying to force them to tell you exactly what dish they'll be bringing to YOUR party. Take it or leave it. That's why it's called potLUCK.