Continuity mistake: During the whack-a-robot game, a big chunk of the broken robot lies on the counter, but it disappears a shot later.
Continuity mistake: Trevor is driving the Ecto-1 and complains that Podcast is breathing on him. We see Podcast very close to him. The shot cuts and Podcast is now further away.
Continuity mistake: When the first witch turns into a mouse, the way her empty dress lies on the table changes between the first and second shot, in a mere frame. Check the plate on the left, for example: it swaps from uncovered to covered by the dress. Also, the first shot shows more lace by the edge of the table, while the second shot shows much more silky fabric.
Revealing mistake: When Millie jumps on her shop teacher's back, the teacher switches from the actor to a stunt double as they begin to tumble about. Noticeably, his hair suddenly gets much darker and has less gray in it. You can also get brief glimpses of his face, and it's obviously not the actor.