I've seen it happen in my game a few times. I usually direct my sims to sit on a chair and that solves it. But it does seem bugged. It only happens at the starlight accolades when all seats are taken I think, but I haven't attended the ceremony in a while.
Not sure if it's related, but there seems to be a general aversion to sitting down. I've noticed sims often keep standing when they eat, even if the dining table is right in front of them. And they also tend to do that when watching television.
It doesn't happen all the time, but enough to notice it.