When Bart spies on people through his telescope, he can hear what they're saying. From his vantage point, that would be impossible.
When Bart falls from his treehouse, his scream lasts for a few seconds. Which would mean that he fell more than 200 feet. Which he obviously didn't.
When Otto gets splashed while driving the pool-mobile, he mentions that he has to replace his back window. Yet, lines are animated to indicate that a window is there.
In the widescreen version as seen on Disney+, Matt Groening's name is misspelled as "Matt Gorening" in the end credits.
When Bart takes his play away from Lisa and puts it in the drawer of his bedside table, the drawer is heard closing but clearly remains open.
Nelson tells Bart that his epidermis is showing, then explains to someone that it means hair. It actually is another word for your skin.
Bart describes his manuscript as a play but the excerpt he reads is in the form of a novel.