Originally Posted by
eLhabib
The game can only record the input of your controller (and of the AI), but the effect in-game runs through the physics engine. Now the problem is that you can't rewind the effect of the physics engine - that's why you can't rewind Halo's saved films. But even for an un-rewindable replay, the physics engine doesn't act exactly the same way every time (like in real life, you just cannot exactly replicate a situation) - that's why in a game with more complicated physics it can mess up the whole replay.