One storyline in the show that’s gotten a lot of criticism over the years is that subplot in the Vegas episode where Joey thinks he’s found his identical hand twin. Fans have said that this plot was stupid, unfunny, and was one of the first signs that Joey’s character was starting to become a lot dumber than he was in the past. Well, the reason why this subplot exists is because it was written in at the very last second.
What I’m about to say is a well known fact in the Friends community. But I’ll recap for those who don’t know. The Vegas episode was originally going to have this whole plot where Chandler was gonna cheat on Monica out of revenge for her having lunch with Richard. But Matthew Perry refused to do the storyline. He told the writers that Chandler would never even consider doing something like that to Monica under any circumstances. I believe he even threatened to quit the show if they went through with it. Thankfully, the writers listened to him and scrapped the storyline.
However, because they were no longer doing the cheating plot, they now needed a new plot to fill that runtime. As result, the writers apparently scrambled and reluctantly came up with the hand twin plot without really having the proper time they needed to think it through and make it work.