When Groundskeeper Willie claims he got arthritis in his fingers, from space invaders in 1977. Wiggum assumes Willie is talking about the arcade game. However, Space Invaders was not released until 1978. (Edited as a character error because Wiggum assumed Willie was talking about the game. Willie appeared to believe actual space invaders are the reason behind the handicap.)
During the concluding episode, when Lisa is explaining to her mother how Homer's prints could have gotten on the weapon, a flashback scene shows Homer drop reaching under his car seat to get something he dropped; he touches various items, including the gun, then right AFTER the gun, he touches the lollipop. Yet at the end of the episode where Mr. Burns recounts, in another flashback, when he was accidently shot by Maggie, he gun and lollipop fall under the seat, but the lollipop falls to the LEFT of the gun.