It happens on occasion, for me most often in the form of getting stuck in an endless chess game even though according to the game "this sim is leaving." Easy fix though; activate cheats ("testingcheats on") shift-click on the sim, and select "reset object (debug)". They should reset and go on their way, most likely waddling from low bladder need depending on how long they've been stuck ;) It's a good idea to turn off cheats after you're done, just in case ("testingcheats off") Hope that helps!