I've found that it is pretty easy to get a physics entity stuck in the level geometry. All you need is the entity to hit a block at a decent enough speed (usually not all that much) and part of it will becom lodged inside the block, becoming stuck and no longer responding to gravity, phent_addcentralforce, etc. I've tried to eliminate as many thin walls from my game as possible, but the entity has even gotten stuck on reasonably thick walls and floors, so im not really sure what to do here...