Revealing mistake: When the crew at Prometheus are viewing the tape of Hulk destroying the helicopter, it is obvious the tape is just stock footage from the scene earlier when the Hulk actually does destroy the helicopter. The camera angles used are exactly the same, and they are also angles the other crew wouldn't be able to film from.
Revealing mistake: David and Del Fry are fighting after David gives Del Fry the shot. When showing the back of Del Fry, his hair is black not gray, revealing that it's a stuntman.
Answer: The Hulk is accused of killing Dr. David Banner and Dr. Elenia Marks. After the lab explodes, McGee sees the Hulk emerge carrying the body of Dr. Marks, so he assumed the creature is guilty. It was McGee who caused the explosion. After following the trail of evidence of the Hulk to the lab, he hides when David and Elania arrive. In a darkened storage closet, he knocks over a bottle of flammable liquid. He confronts them, then is forced to leave by Banner, who says, "Don't make me angry, you wouldn't like me when I'm angry." When Banner reenters the lab, it explodes, caused by the Hulk when it went on a rampage trashing the lab, causing many electrical systems to catch fire, thus igniting the flammable liquid.