2. Marge and Homer's Changing Backstory
One of the most egregious examples of the series' retconning came in the Season 19 episode "That 90's Show", in which Marge attends Springfield University in the 90's and Homer quits his job at his father's laser tag warehouse to become a wildly successful grunge singer. His band "Sadgasm" score a big chart hit "Shave Me," but glorious achievements aside, this blatantly contradicts the third season episode "I Married Marge"(as well as many other episodes) where Homer worked at a mini-golf course in the 80's and knocked up Marge with Bart there after hours, after their romantic date of watching The Empire Strikes Back. To the show's credit, this bizarre alternate timeline has never been mentioned again. Here's hoping it stays that way.