The white sheet Smithers puts on the chair for Mr. Burns to sit on turns orange when he stands up again.
When Marge's Ringo Starr portrait is rejected in high school, the background is red, but when Marge sends it to Ringo, the background in it is blue.
Ringo's typewriter carriage travels in the opposite direction from how it should. In order to type letters left to right, the carriage moves right to left and has to get pushed back into place from the left.
Mr. Burns' office has a balcony, but no door to get to it, just windows.
After the donut delivery guy asks Carl why all the donuts are piling up, Carl responds with Lenny's voice.
Marge sent Ringo Starr the painting of him long before she started living with Homer. If this is the case, how did Ringo know where to send his reply?
When Homer is rescued from the water slide he's stuck in all the kids who trapped in with him there have disappeared.