TF2 has a problem with hit detection in mid-air. The hitboxes seem to travel a bit faster than the models, so you have to lead your shots a bit, even with hit scan stuff. It seems to have a similar problem with player hull collisions.http://youtu.be/TGjd-ARy0gk?hd=1
cmon mayne, i thought only hitscan was supposed to have this problem. Never had it this bad with projectiles before.
There's a thread on SPUF about porting TF2 to the Portal 2/CS:GO version of the source engine and it has replies from Valve employees; so maybe they'll actually do it!*
*Probably not.