Continuity mistake: When the Hulk throws the tire during his second transformation, the first shot shows a horizontal trajectory but the second shot shows a 45 degree angle trajectory, which makes the tire land much further than expected.
Continuity mistake: When David is leaving at the end, he is wearing a blue shirt under his coat. Moments later, he removes his coat but he is wearing a different color shirt with a square motif.
Continuity mistake: When the Hulk throws Mr. Malone in the river during his second transformation, the first shot is too long and we see that he would fall a few feet from the closest river shore. But in the second shot he is flying like crazy and almost lands on the remote river shore.
Continuity mistake: After the Hulk breaks the wall to escape at the end of his second transformation, he is wearing black pants instead of his usual beige pants.
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.
Continuity mistake: Before his first transformation, David is wearing a white and blue striped shirt with blue jeans. During his transformation he is wearing a red and blue square motif shirt with brown pants. When fully transformed into the Hulk, he is wearing blue jeans again. But when he's removing the rocks to free the professor seconds later, he is wearing a different kind of brown pants. Finally he's wearing blue jeans again for the rest of this first transformation.
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: During the first transformation, just before the Hulk breaks the wall, we can see the green shoes he is wearing to protect his feet.
Continuity mistake: While Dr. Morrow is saying, "I did some preliminary tests on David. The limbic system is definitely affected," he is holding a folder containing patient information and a picture. In the next shot the folder contains a different picture with no time to have turned any pages.
Continuity mistake: When David is on the hospital bed before his first transformation, he is wearing beige pants. During his transformation he is wearing light blue pants, but when fully transformed into the Hulk he is wearing beige pants again.
Continuity mistake: After David's second transformation, we can see the dynamite sticks on the floor, tied with a piece of black tape at the top and another piece of tape much further at the bottom. When the Hulk takes the tied dynamite sticks in his hands, a third piece of black tape appears close to the existing piece on the top of the sticks.
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).
Blind Rage - S3-E2
Revealing mistake: While pushing back the tank, the Hulk seems to be wearing light green loafers.
Brain Child - S3-E3
Continuity mistake: When David leaves at the end he's wearing beige pants, but in the next shot he's wearing dark pants.
Visible crew/equipment: When Colleen is driving cab you can see somebody's fingers on the seat behind her.
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: An illness, although it's never stated what kind of illness. David blames his dad for her death because his father never took her to the hospital. So in all likelihood, her illness was never diagnosed anyways and David might not even have known what it was.
Bishop73