Diary of a Wimpy Kid/WMG

The books are an exaggeration, and the movies are the true story
The books have Greg be less of a Jerkass and makes the tales of abuse even more abusive. He also cuts parts he doesn't like (like getting beaten in wrestling by Patty Farrel). The movies are the true story of what happened. Example: In the book, Greg frightened the little children with a worm. In the movie, Greg says that aliens are going to eat them, and abandons them in a construction pit. IN THE RAIN.
 * Actually, he's more of a dick in the book; in the movie he had a reason to do that (He thought a bully from earlier was approaching)
 * Yeah, he's a much nicer guy in the movies - I'd call the movies the exaggeration.

Manny will grow up to become a Psychopathic Manchild Serial Killer
As of book 6, Manny is starting to show an alarming Lack of Empathy and enough boldness to start openly bullying the other members of his family. He's standing right on the edge of Creepy Child territory, and, in light of this, it wouldn't be all that surprising to see him wind up as a temper-tantrum-throwing, thumb-sucking murderer a few years down the line.
 * The first several books do show that Manny's parents spoil him incredibly rotten, much to Greg's chagrin.

The books are actually complex social commentary on the state of America's educational system.
Yes, a teacher not knowing how "jolly" is correctly spelled and an eighth grader not knowing how to carry in math could both be chalked up to Rule of Funny, but some of the events in Greg's school hit too close to home for many students for it all to be exaggeration.
 * Not just the school system. The entire culture of childrearing of today. Think about how Manny is being babied. And Greg's comments on how Rowley's parents shouldn't give him prizes just to feel like winner. And Susan, the Moral Guardian.

Manny is autistic
The whole hot dog issue in Cabin Fever, plus he doesn't seem to be maturing. Not to mention he's young but smart enough to turn off power to the entire house except his own room.

Greg's Dad spent the blizzard in the hotel with his mistress
Dunno, it just seemed odd that he would say he couldn't get home.

Greg (in the movie at least) was the one who wrote on the door
The handwriting on the wall matches up with Greg's. In the book where it shows a cartoon he drew of the incident, this can be easily explained as artistic interpretation. In the movie, however, the only explanation is Greg did it. Perhaps something illicit was brought to the party and Greg imbibed. Under the influence, Greg started to think his brother was a really cool guy and thought it would be nice to write "Roderick rules!" on the door in black magic marker. He has no memory of this event.

Frank (Greg's dad) thinks that Greg is gay and does not approve.
Greg acts really feminine and flamboyant at times; caring for a baby doll as if it were his own child, saying that he'd choose water jazz over swim team, not liking sports and other "manly" things, etc. Perhaps Frank suspects that Greg is gay, and is trying to "stamp it out of him," hence why he throws away said doll without asking, refuses to listen when Greg expresses dislike of sports, tries to send him to military school, and other things like that.

Manny is actually a very smart kid.
He's definitely smart enough to know how to work the electricity in the house and appears to know that his mom accepts "I'm only thwee!" as an excuse. He's simply Obfuscating Stupidity and pretends to not be maturing because he knows that as long as he acts childish and throws tantrums, he'll always get his way. Manny, you Magnificent Bastard.