It's the Roseanne ending, where it turns out the events of that season never actually happened.
Yeah, this. I'll explain in more detail. *Roseanne spoilers*
Basically, during Roseanna, around the end, a character has a heart attack at a wedding, but he survives. The family then goes on to win the lottery. There are two daughters in the show, Becky and Darlenne, and they each have their husbands. Becky has Mark and Darlenne has David.
Roseanne also has a sister, Jackie, who is known for sleeping around with men a lot.
This is how the viewer is told the story went, literally until about the last 5 minutes of the show. At that point, it is revealed that the show is a novel Roseanne is writing about her life. However, she changed a lot of things. The big ones are that her husband, Dan, did not actually survive the heart attack at the wedding. The family also never won the lottery.
Furthermore, she reveals that she swapped her daughters' boyfriends, later husbands. Darlenne was with Mark in reality, while Becky and David were together. Finally, in regards to her sister, Jackie turned out to be gay. Roseanne, in her book, continued to write her as she always did.
Again, this is all revealed in the final moments of the show. There is absolutely no indication that this was happening beforehand. Thus, if you watched the show to its conclusion, it's one of the biggest "wtf" moments you will ever have experienced while watching a tv show.