Continuity mistake: When Adler moves David into the barrel and loads him into the truck before his first transformation, David's head is at the bottom of the barrel. But when the Hulk emerges from the barrel he is already standing on his feet.
Continuity mistake: When Mike is on his balcony with Linda, we see that he is talking to 12 workers. But, in the next shot, a 13th worker wearing an orange hat and an olive green jacket magically appears almost completely to the right. However, when the 12 workers leave the area, the 13th worker cannot be seen anywhere.
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