Thought of a new one today.
A character or characters are complete idiots, or have no talent in anything whatsoever but due to incredible misconceptions is thought to be incredibly smart or very dangerous.
Beavis and Butt-Head Do America!
Ha ha, yes!
I thought of some more:
The ugly girl gets the cute guy...except the ugly girl isn't actually ugly. (Where they throw on a pair of glasses and some braces, and call the girl ugly).
The girl gets the guy even though they're not compatible...except they actually are compatible. (basically the same as the ugly girl one).
Everyone's straight in kids movies. (heterosexual relationships are often portrayed in kids movies, though usually only romantically, but homosexual relationships never are.) This is especially true in Disney movies.
(This one kind of pisses me off, because there's no logical explanation for it.)
Everything conveniently falls in place. (events that are extremely improbably just happen to happen to drive the plot of the movie.)
Oh, another one is where something bad happens and they immediately jump to the conclusion that the child is responsible, and berate them for it. (Hide & Seek and Don't Be Afraid of the Dark are two examples, but I'm sure there are more)