The $5 bill on Cory's lunch tray disappears and reappears between shots.
The show takes place in Philadelphia in mid-December, but none of the characters are wearing coats, and the trees on the outdoor sets still have leaves.
When Mr. Feeny reads A Christmas Carol, he is saying the ending, but the book is open to the beginning.
When the students are leaving the classroom to start the winter vacation the boom mic is visible at the top of the door frame.