Today, for fun, I tried to push a box with a combine corpse on top of it into the elevator after you first meet Alyx. I could not do it, so in the end I gave up and went into the elevator. Alyx said somestuff and started the elevator. However, the corpses leg was poking through and prevented the elevator from going down, so I was stuck their. I didn't think the physics engin affected the game THAT much... wow... this game continues to amaze me after a month or so of playing.