Well, as I said, this bug is really more a legion of bugs than one specific problem. The basic idea is that any chair or sofa or whatever can get stuck in use somehow. Usually, this is how it happens:
Ah! Okay, got it.
With regard to the desk chair, I think it must be a play catch, or maybe a snowball fight social that didn't complete. The seat is in front of a computer which most often gets used to play SSX, then some random housemate wants to go play outside. Sometimes, I think they don't make it.
Anyway, I'll keep an eye out the next time I play. Of course, that means that now I'm looking the problem will fix itself.