Continuity mistake: When Hulk throws the chunk of sheet metal at the helicopter, it glances off the rotor housing (taking a ragged piece with it.) When the chopper crashes, however, the propeller comes off with a perfectly clean edge, as if the projectile had sheared completely through the base.
Continuity mistake: After the first time we see Bruce transform into the Hulk, he has hitched a ride with the elderly man. You will remember he has 3 1/4 length pants on with a few rips. It cuts to a quick clip of him walking up the road but this time he has full length pants on. They switch back to shorter length pants straight after.
Continuity mistake: When Dr. Ross empties her handbag on the bed, there is a Givenchy compact with the Givenchy insignia face up. When the bed is shown again, the compact is face down.
Revealing mistake: In the beginning of the film, Bruce Banner's hair is a dark brown and looks natural. When he is doing the anger control exercises, he is obviously wearing a black wig which is not natural looking at all. This happens a few times during the movie - it switches from his natural hair to the very dark wig. And at times, even his eyebrows look much darker and thicker.
Factual error: When Blonsky is seen wearing the American Class A uniform, it's visible he has not shaved his face for days. Quite unthinkable for the military - be it American or British. If he had time to change from his BDUs to the Class A uniform then he would have had time to shave as well. Soldiers in Iraq aren't even excused from shaving daily unless they have a medical condition that prevents them from shaving.