Revealing mistake: During the first transformation, just before the Hulk breaks the wall, we can see the green shoes he is wearing to protect his feet.
Revealing mistake: When the Hulk throws the large bell to break the tower's wall during his second transformation, we can see three cables attached to the bell, most likely used to guide its trajectory.
Revealing mistake: When Adler fights with David close to the detonator just before his second transformation, the shots are filmed too close. It makes it really obvious that a stuntman is replacing David for most of the fight.
Revealing mistake: When the Hulk is inside the steam room and he throws his fists through the door, he loses fairly big patches of green make-up on both forearms in the process. (There is no CGI used in this 70s era TV show.)
Revealing mistake: Near the end of the episode, after the Hulk jumps from the wooden crate, he walks around barefoot. But right after the pickup hits the car, we can see that the Hulk is wearing green shoes to protect his feet.
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.
Blind Rage - S3-E2
Revealing mistake: During his first transformation, the Hulk gets behind two soldiers and smiles twice. His second smile is too broad and reveals a very shiny silver tooth filling on his right lower jaw. David does have fillings but they do not cover the external area of the teeth (Ref: when he hugs Katie at the end of the 2nd part of Prometheus, right after his last transformation).
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: When the Hulk is lifting his left foot at the beginning of his first transformation to break free from the "leg shackle", there is a close shot where we should see his toes. But we cannot see them because he is wearing some green pantyhose with brown soles to protect his feet.
Revealing mistake: When The Hulk tosses the abusive father across the gym his blond hair looks fluffier and darker, indicating it's a wig on a stuntman.
Another Path - S2-E7
Revealing mistake: Mako's hair is obviously a wig, it's most noticeable towards the end, in the close ups when he's fighting the other man.
Blind Rage - S3-E2
Revealing mistake: While pushing back the tank, the Hulk seems to be wearing light green loafers.
Chosen answer: Bruce has always been the character's middle name. In the comics he is Robert Bruce Banner, here he is David Bruce Banner. Apparently the executives behind the show preferred the name David.