Not sure if it's a bug, but it seems like one. If, for example, my Sim has a costume party, the non-playing Sims leave their costumes on after they've gone home ... LONG after they've gone home. In fact, they stay like that for the rest of the game. Trying to flirt with a Sim dressed as a hot-dog is a tad weird. Same for black and white and pool parties. NPCs don't change their clothes afterwards.