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.  
