When Bart sneaks into the cinema, he forgets to take his skateboard in with him and doesn't retrieve it after he's ejected. It reappears in subsequent episodes without explanation.
The first time Bart slides down the stairs (in church clothes), it looks as if one the animation cells has a hairline crack it in. It can be seen on the left of the screen.
When the boys sneak into Space Mutants 4 there is a line going around the block to get in, during the movie, the theatre is almost empty. This is only a one screen theatre.
When Bart is cutting off the head of the statue, he is sitting on the statue's right shoulder. But Bart doesn't appear in the shadow of the statue as the camera pans out.
The theatre marquee that said "Space Mutants IV: The Trilogy Continues"
was a contradictory title for a series that was supposed to be only three movies in it.
When Bart is preparing to cut the statue's head off he holds up a hacksaw, which suddenly vanishes a second later (approx 14:42).
The colors on the desert island painting above the Simpson couch are switched. The island itself is colored in blue, and the sea is colored in a sandy color.
When the angry mob is chasing Homer and Bart through the streets, in one shot, Dr. Marvin Monroe appears to be animated twice.
After the boys leave the Kwik-E-Mart, Bart asks the bullies where they got all the stuff. Jumbo replies, "5 finger discount, man." Every character on the Simpsons only has 4 fingers on each hand.