Continuity mistake: While Bart starts the I.Q. quiz and is reading it out loud, his teacher says "Shh. Visualize it Bart." While this happens if you look in the background you can see the alphabet line. It starts out Tt Uu Vv Ww, then Bart's teacher blocks out some, then you see the letter T again.
Continuity mistake: When Homer picks up the yellow book from under the settee, you can see there is a white label on the front cover. But when Lisa is reading the book, the label is now on the back cover, and when we see Lisa a few shots later, it is back on the front again.