I've noticed some of the scripted sequences have a fixed amount of steps, instead of actually walking to the point they're suppose to perform the action.
Such as when you first get the Grav-Gun, in the lab before hand, Alyx's father (I don't know why I can't remember his name right this moment), got pushed out of the way by me, and started typing in mid air.
Another thing was during the grav-gun course, I dropped a box on Alyx and she disappeared. I found her at the beginning of the next section. And once the training was over DOG got stuck outside the airlock and magically appeared inside the airlock.
So, I think some of their timing is off. (I didn't try launching the ball over the cliff though.)
What's this? I can't hack apart the quote for length!? Bah!
Anyway -- I actually launched the ball over the cliff the first time I went through the chapter -- I, erm, did it by acident
First time I used the gravity gun, I dropped a paint can on my head 
[addsig]