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.
Factual error: The story takes place in Galveston, Texas. The geographical area is flat, but if you look closely near the last hulk out, there are mountains in the background.
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