“Sick at a formal/important event” will always be the top, I think, but “sick at a celebration” is also so so good. Someone trying their absolute best to be cheerful and have a good time and eat and drink and they just feel so miserable the whole time. Maybe they feel bad for bailing, too, because true celebrations of something are so rare sometimes. They want to tough it out and want to honestly enjoy some of it. Everyone’s earned that, right?
A bonus variation of this is if the process of working so hard for something causes them to get sick once they’ve finally reached the goal. Sick for their degree conferral, or for some type of accolade or recognition event. They worked themself too hard for it and their body’s catching up with them. Everyone around them congratulating them while thinking woah, you don’t look so good.
