It is a well-known fact that Friends isn't the most consistent of shows where continuity's concerned. From small details like characters' birthdays changing to full-blown cases of characters disappearing (what ever did happen to Ross's son Ben?). Well, eagle-eyed Friends fans think they've spotted a new one.
In the season six episode, The One With Ross's Denial, Monica and Chandler argue over Rachel's newly vacated bedroom. Chandler wants to deck the room out like an old arcade, with classic games like Asteroids and Space Invaders. Monica shatters his dream, saying: "Interesting idea, talk about it, but no. The beautiful guest room’s gonna be filled with antiques."

However, come season eight Monica drastically changes her tune. Not only does she decide she likes the idea of an arcade game in the apartment, but she even suggests putting it in the guest bedroom! This rapid change of heart comes when Pheobe buys Monica and Chandler a Ms Pac-Man machine as a wedding gift. Miraculously, Monica remembers how she used to love arcade games as a child, exclaiming: "I practically spent my childhood at the arcade!"
Wait, where did this new Monica come from? Did she somehow forget that only two seasons ago she hated the idea of an arcade game in the house?

Monica becomes so obsessed with the game that she even turns hostile towards Pheobe when she starts getting good at it. She apologizes, explaining that it's because she: "loves the game so much." Seems like a pretty drastic attitude change from season six. Of course, given Friends' track history, it's totally plausible that the writers simply forgot about Monica's previous attitude towards arcade games.
It wouldn't be the first time the characters in Friends suffered from apparent memory loss. A constant source of annoyance for Friends fans is found at the very beginning of the series when Rachel and Chandler act as if they don't know one another. This is particularly strange as it is later revealed that they have known each other since they were teenagers, and even once shared a kiss!

And let's make sure we don't forget Ross's birthday this year - October 18th, remember? Or is it in December? That's what he told Gunther! The show's abundance of plot holes and continuity errors are so well known that Friends fans have taken to Twitter to laugh about it.
One user joked: "Friends plot hole: Joey and Chandler have a TV even though they live in a universe where Friends doesn't exist so there's no point."
Another took aim at Ross, tweeting: "Ross is the worst friend but had the most game which is the biggest plot hole in Friends."
At this point, the only sensible thing we can all do is rewatch the entire series AGAIN and finally get to the bottom of these mysteries.