The fast and loose continuity of The Simpsons has allowed for plenty of experimentation and revelations over the years, with varying backstories that define core relationships within the series. But all the way back in Season 12, The Simpsons almost changed everything with a single episode. "Insane Clown Poppy" almost introduced one of the most radical retcons possible — that Homer had a child before beginning his relationship with Marge, resulting in a fourth Simpson kid.
While attending a town book fair to get an autograph from Krusty the Clown, Bart finds himself standing next to a girl named Sophie who reveals to the clown that she's actually his daughter. During a USO tour years earlier, Krusty spent a night with a soldier named Erin, which resulted in the birth of Sophie. The episode focuses on Krusty and Sophie's relationship, culminating in Homer and Krusty doing their best to recover her beloved violin after Krusty loses it to Fat Tony in a poker game.
According to the Season 12 DVD commentary for the episode, the story was originally based on a spec-script Frink and Payne had written years previously as a proposed episode of The Critic, another animated series by Simpsons veterans Al Jean and Mike Reiss. After that show was canceled, Frink and Payne were brought onto The Simpsons and adapted the concept. Their original idea for the script didn't focus on Krusty, however, but rather on Homer. The episode would have revealed that Homer had unknowingly impregnated a woman he briefly dated before beginning his relationship with Marge, and would have shown the child confronting their father for the first time. Then-showrunner Mike Skully suggested changing the plot to center around Krusty instead of Homer, leading to "Insane Clown Poppy."
It's a good thing he did so, too — while the twelfth season might have come at a time of experimentation for The Simpsons (with successful episodes like the perspective-splitting "Trilogy of Errors" and the Flowers for Algernon-inspired "HOMR"), the revelation of Homer's unknown child would have thrown the show's core relationships into disarray. It would have severely undercut the established history of the show, which has consistently painted Homer and Marge as true loves since their teenage years who've only been intimate with each other. It would have upended the relationship between Homer and his children, throwing a powerful new wrench into the family by introducing a brand new oldest Simpsons child.
Considering that Homer's genuine attempts at fatherhood are some of his only redeeming qualities — especially during this period of the show when he'd transitioned into "Jerkass Homer" — it would have fundamentally altered the character in a negative way. Other episodes that introduced lesser but still impactful retcons to continuity (such as Season 9's "The Principal and the Pauper," which changed the history of Seymour Skinner) were met with open distaste. The existence of a secret Simpson would have likely garnered an even fiercer reaction, especially if the character retreated into obscurity (much as Sophie has, with the character only appearing in a handful of recent episodes) instead of reflecting how much of a monumentally impactful change that would be.
Notably, the Season 24 episode "Adventures In Baby Getting" revealed that Homer had donated sperm as an extra source of income while still dating Marge, leading to a number of children being born with his DNA. But his relationship and commitment to Marge are still at the center of that storyline instead of revealing a never-before mentioned other woman, and there's no revelation of a "missing Simpson."