As a long-running sitcom, it’s inevitable that Friends would mess up its continuity from time to time, but the beloved series actually offers a solution to one particular plot hole involving Rachel and Chandler having supposedly met "for the first time" twice in the show. It may seem a contradiction to accept they managed to do so on multiple occasions, but it actually fits their characters for them not to know it.
Famous for launching the careers of Jennifer Aniston, Courtney Cox, David Schwimmer, Matthew Perry, and Lisa Kudrow, the '90s sitcom Friends remains one of the most iconic hang-out shows ever made and continues to influence new additions to the genre to this day. Without the hilarious misadventures and tumultuous love lives of the Central Perk quintet, there would be no how much money Joey owed Chandler - and did occasionally create some plot holes in its 10 season run.
For example, in an episode that sees the group recounting their worst Thanksgivings, the appropriately titled “The One With All The Thanksgivings”, there's a flashback of Rachel’s which features her and Chandler meeting for the first time on Thanksgiving. It’s a scene that provides a few laughs and, like most of the show’s flashbacks, hasn’t aged all that well thanks to a few mean-spirited gags mocking Monica’s weight. But there are two other episodes that also depict Rachel and Chandler meeting for the "first time” - and while that may seem that something went wrong, it's key to that the Thanksgiving episode establishes that Chandler and Rachel were vastly different characters in the past, and wouldn't necessarily notice certain people because of their very nature. On top of that, according to Kevin Bright on the episode’s commentary, this Friends plot hole isn’t an error, but rather a case of the characters involved mising things.
Much as the episode’s flashback appears to be just another Friends mistake, there’s actually a reasonable explanation put forward by one of the creators of Friends, and it’s an answer that displays the show’s eventual influence on the next generation of sitcoms. Creator Bright says that Chandler and Rachel didn't recognize one another because they looked so different, but it's more than that - Friends shows both Chandler and Rachel on journeys that make them less self-absorbed. At the time of their first meetings, they were still shallow people and it fits that they didn't one another, prompting Rachel's claim that this was their first meeting. The younger Rachel, meanwhile, simply wouldn't pay attention to a dorky friend of her best friend's nerdy brother.
The idea of Rachel and Chandler not recognizing one another speaks to both Chandler being so superficial that all he saw was Monica's hot friend when they were young and Rachel dismissing him entirely because she was similarly superficial. Friends saw both characters grow into more rounded and likable figures over the years, meaning they eventually weren’t quite so surface-level by the end of the series. This tactic of using the character’s imperfect, biased recollections of events for comedic effect was one that formed the premise of the later Friends-inspired hang-out sitcom How I Met Your Mother. That noughties show took this idea up a notch, basing entire episodes around the idea that each character's version of the past was colored by their perspective and couldn’t necessarily be trusted as accurate. As effectively (and occasionally badly) as How I Met Your Mother used the trick, though, the show has Friends to thank for pioneering the method of using different character’s mixed memories to illustrate their growth (and pave over problematic plot holes).