6. Homer's Sterility

Marge takes birth control in the 21st season episode "O Brother, Where Bart Thou?". We see Bart replace her pills with Tic Tacs in his attempt to get a little brother. However, the show has firmly established since the 3rd season episode "Brother, Can You Spare Two Dimes" another long-running gag that Homer has been rendered irrevocably sterile by his prolonged radiation exposure at Springfield Nuclear Power Plant, so this seems like an uncharacteristically futile effort from Bart. In a more recent example, Marge's desire to have a fourth child was the plot point behind last year's episode "Adventures in Baby Getting", which is rather baffling if Homer's still shooting blanks.