Deliberate mistake: In the scene where Forrest is describing his heritage, the clip of KKK on horses is taken from The Birth of a Nation. In this clip, you can see tire tracks on the dirt road well before automobiles were supposed to be in existence. Robert Zemeckis decided to leave this mistake in to perfectly match the clip to the original movie.
Forrest Gump (1994)
1 deliberate mistake - chronological order
Directed by: Robert Zemeckis
Starring: Tom Hanks, Sally Field, Gary Sinise, Robin Wright, Robin Wright Penn
Revealing mistake: When Forrest finally catches shrimp on the boat, the net drops shrimp on the deck which are not only already dead, they're headless and very obviously processed for shipment to market. (01:32:30)
Forrest Gump: Bit me directly in the but-tocks. They said it was a million dollar wound, but the Army must keep that money, 'cause I still ain't seen a nickel of that million dollars.
Trivia: Forrest's son is played by Haley Joel Osment, who later achieved great fame as the troubled Cole in the "The Sixth Sense". (02:01:15)
Question: What does Jenny die of? And is it mentioned anywhere in the film at all?
Answer: In 1981, when Forrest meets Jenny again, she says she has a virus that doctors don't know. This virus was the HIV virus, as 1981 marks the AIDS outbreak in the World.
Answer: It could have been HIV or Hepatitis C or something else. Maybe someone related to the story or even Winston Groom himself told this at some other place. Nonetheless, it's false that it appears in that book.
It absolutely appears in the second book, 'Gump and Company'.
Answer: It's not specified in the film, but in the sequel book "Gump & Co" the author mentions that Jenny dies from Hepatitis C as a former drug addict in the early 70s. Hepatitis C was an unknown disease until 1989.