Forrest Gump/WMG

Forrest had Aspergers or Autism
Based on how he's smart enough to build a gun in record time and be a champion at ping pong, he's actually a genius, but he lacks any social skills.

All of Tom Hanks' non Gump roles are one of Forrest's stories
See How Saving Private Ryan Should Have Ended.
 * Oh no, Forrest is trapped on a island! Now he's in love with Meg Ryan! Now he's a toy in some kid's bedroom! Excellent!

Mama Gump was a prostitute.
Forrest didn't know his father, she slept with the principal to get him into public school, and Forrest said that they got money from folks passing by. There's also a theory that men are attracted to women who remind them of their mothers. Jenny was easy, and this could be why Forrest subconsciously was in love with Jenny.

John Lennon divorced Yoko Ono in this film's universe
Because Forrest is using the talk show chair that, in Real Life, Yoko Ono used. In this universe, John and Yoko weren't as inseparable early on, which meant that their marriage didn't survive the Lost Weekend.

This changed greater history less than it should've. There are a few more Lennon albums, but nothing as memorable as Plastic Ono Band or Imagine; and while John and Paul did work together more, there were no album-length collaborations. And John still died "on schedule."


 * How do you explain John quoting Imagine during the interview?
 * He wasn't quoting Imagine, Forrest was giving him the idea for it.
 * If John was separated from Yoko, there would have been no need for him to be out at the recording studio on December 8th 1980 recording with Yoko. How would he still be killed "on schedule"?
 * Forrest recollects that John was killed "while going to see his kid." So maybe he was just in New York, doing who-knows-what(hell, he could've been in New York meeting the other Beatles, as they were in talks at the time about a possible reunion tour or two) and was returning to his apartment when some asshole gunned him down.
 * John's death was referenced in the film. Even if it wasn't on schedule, it was before Forrest tells his story.

Forrest is Booster Gold
Forrest is responsible for a ton of important historical events and is generally assumed to be an idiot when he isn't leading the world. Booster Gold's job is to protect the timestream; to accomplish this, he has to pretend to be an idiot so that no-one will get suspicious.
 * Not to mention both are star football players.

Forrest (Both of them) has AIDS (or at least HIV)
Jenny died of an incurable virus, presumably HIV or AIDS, which was most likely gained from her countercultural lifestyle. HIV is an STD, and can also be transmitted to unborn children. Thus, both Forrests could catch AIDS and its risk of dying. How's that for a Downer Ending?
 * It gets better. See the related WMG in Poison Oak Epileptic Trees...

Forrest is a Grim Reaper, but he is not aware of it
He brought death to the Kennedys, Bubba, and John Lennon, and changed many other people's lives. He made Lt. Dan realize his hour had not come yet. He spent the better part of his life trying to convince Jenny to live in his house; she died right after she agreed to do so. The only person that realized about this was his momma near the end of her life, hence her final words being that she didn't know she was destined to be his mom and that she had accepted death as a part of life.
 * While normally depicted as a skeleton, the Grim Reaper is a fresh, sturdy set of bones, never aging. After maturing into a young man, Forest does not age either even though at least two decades pass.

Forrest wasn't happy, either.
Think about it. Even though Forrest paid a lot of attention to them, he never thought much of his accomplishments. He just went with them; it was clear he didn't exactly enjoy them.

On top of that, he barely ever smiled. He was only happy when he was with his friends or his love.

This could give the movie a new message: while indulging in counter-cultural lifestyle brings no happiness, neither does a life like Forrest's. So the meaning could be that people need each other to be happy.
 * Best WMG yet.
 * Maybe it's just because he's not bright enough to understand what he's achieved?

The story was a fabricaion of Forrest's to impress people at the bus stop.
He's just a good storyteller. He inserted himself into historical events to impress people. Jenny treating him like dirt is part of his Revenge Fic, even though he still loves her and wants to meet her.

This explains how he tells the scenes he wasn't there for.
 * As stated below, there was an article in Fortune, so he had to have done something.

Forrest Gump genuinely believed those memories were real, but his mental retardation distorted them.
It's obvious that Jenny treated him like dirt, so he may have altered his memories of Jenny to show her in a more positive light. She may have truly abused him.

He naturally exaggerated many of his other memories, making them seem more incredible than they really are: for instance, thinking he ran across the United States when he just ran around the county a couple of times, or meeting Elvis when he just met a drunk hillbilly.

There is some truth to his memories. He did become a very rich man; there was an article on him in Fortune, so there was no way he didn't. He really did meet Jenny. But the truth has been distorted by his unreliable memory.

Forrest is in a mental home; this is all just a dream.
A man like Forrest wouldn't have had the freedom a normal human being would have. Most people of his intellectual capacity would be in a mental home or under legal guardianship. Forrest isn't under legal guardianship. So this is probably just a series of dreams he's had.

Who'd expect a mentally handicapped man to be a war hero, table tennis champion, millionare in the shrimp industry, and cross country runner?

Forrest Junior is not Forrest Senior's biological son.
Really, this one should be obvious.

Jenny slept around a lot. Every man she was with other than Forrest used, abused, and abandoned her. She knew that Forrest was the one guy who was gullible enough to believe her, and she knew that he had plenty of money. (There was an article on him in Fortune, so there was no way he didn't.) She didn't contact Forrest until her son was several years old and after she learned about her illness.

She may have been pregnant when she showed up at Forrest's house. We never see how long she stayed with him. The only reason she went back to him for that short time was to provide a plausible conception date. Since this is Forrest Gump we're talking about, that could be any time before she started showing.

Jenny even said that her son was "very smart" and did well in school. Intelligence is hereditary. It is improbable that a man with an IQ of 75 could father a boy with above-average intellect.
 * Ah, genetics don't work that way. It's more probable than you think.
 * Going by the WMG of Mama being a prostitute, she could've smoked cigarettes or weed, drank, or taken other substances while pregnant.
 * That would explain why they were a little on the po' side.

Forrest Junior really is Forrest Senior's biological son.
The time period between Forrest and Jenny having sex and Forrest meeting his son does match up. The dates between the two events are fairly vague, so their son could have been conceived, and grown and developed, in that time frame.

Forrest's son also took to him quickly. Natural Father-Son instinct?

Forrest is an alien from space.
There was very little mentioned about Forrest's father, and Forrest's behaviour is... strange.

This would explain his unusual strength (rushes several soldiers to safety under enemy fire), fast reflexes (Ping Pong), fast running (Football Team) and incredible endurance (runs the length of America for 3 years). As well as his very odd mannerisms.

Possible reasons:
 * An alien impregnated his mother, causing for there to be a human/alien hybrid.
 * Forrest was a humanoid alien baby given to Forrest's mother. Possibly the last of it's kind, as a refugee of a dying race.
 * Forrest is an alien trapped in the form of a human baby, with it's memories wiped.

Forrest is an Ridiculously Human Robot, built by his "mother"
Everything said in the previous WMG fits here. Plus the fact he doesn't age once he ceases to be a child. That's because he never grew, there were two different Robot Forrests and Momma somehow managed to replace the fist with the second without arising too much suspicions. And the brackets were a temporary measure while Momma was working to fix the problem with Forrest-1's legs.

Up to Eleven: A very distant ancestor of Forrest was an anserine bird.
All jokes about "bird brains" aside, check out a few imprinting behaviours. Also, geese and ducks are great distance flyers - what better trait for an athlete! As for his strength, well, he's just got the mutant gene working for him.

And maybe all the X Men are descended from that same goose!

The whole story is an adaptation of Candide
I think this says it all.

The missing Mr. Gump Sr. is an Illuminatus.
Forrest never meets his father, and conveniently happens to rub elbows with some very influential people and unwittingly influence things like the start of the rock genre and Watergate. Mr. Gump Sr. (if that is indeed his real name) is subtly puppet-mastering events from behind the scenes and using his son as a pawn. When Forrest gets older, he goes to Europe and ends up helping unravel daddy's Ancient Conspiracy.

Forrest was a test subject for a secret Super Soldier program.
Forrest would have been born right around the end of World War II, and happened to be used as a test subject for a top-secret program to engineer super-soldiers. His mom didn't like to talk about his father because there really wasn't one. While the project had some unfortunate side-effects (spine deformity, mental issues), it did manage to produce a soldier with beyond-Olympic-level speed and stamina, obedient, quick competent at mechanical tasks, and unswervingly loyal to his comrades-in arms, displaying valor above and beyond excellence in the face of heavy enemy (and friendly) fire, as evidenced by said subject being awarded the Congressional Medal of Honor.

Forrest's dad died overseas during World War II.
Just an idea, based on Forrest's age: His dad died in the war before his wife (or girlfriend) could tell him that he was gonna be a father.