So I am at this abandon gas station killing Radscorpins when I notice one of them is part way in the ground and not moving. So I shoot its tail and it falls right through the world (Kill cam chased it under the world)! I love to role play but that mob falling through the world totally killed the immersion for me. So I decided to take a break and visit the forums for a few and try again.
P.S. if you ask me that's a major bug that should have been caught in beta. I realize that the game is a huge open world (even though its not as big as FO3) but come on! People pay good money for your product and expect to have a working product at release. Not some hack job game that your going to be patching for the next 6 months!
Welcome to Bethesda's game engine... things have been dying and falling through the world (or ending up inside models like rocks) since the original release of Oblivion back in 2006.
Generally, it's caused by a bad navigation mesh that allows the actor to clip through the terrain. Combine that with the physics model that flings the corpse around when it dies and you end up with some or all of the actor clipping through scenery and if the center point falls below the surface, down down down goes the actor. Or you'll get situations where part of the actor skeleton gets accelerated to ridiculous speed, giving you the crazy ball of deadly yarn that spins around and gyrates madly, bouncing off walls until it flings off into the far distance.
It won't get better until Bethesda rewrites major portions of the engine or switches to an entirely new engine.