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.
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: 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 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 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.
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