The sign of a good TV show? The fact that more than 10 years after it ended (sob), we're all still talking about it. And thinking about it. And noticing plot holes like these.
1. The gang's WILDLY inconsistent ages. Either there's some kind of weird time portal in Central Perk, or they're just hella forgetful, but the Friends gang can NOT keep track of their ages. For example, in Season 7, Rachel is shown to be the last member of the group to turn 30. However, according to "The One With The Birth" in Season 1, Joey is the youngest at 25 (Twenty-five! They were so YOUNG). Oh, and there are various references to Ross being 29 in seasons 3, 4 AND 5, despite multiple Thanksgivings, Christmases, and other events that prove time has passed. Geller really doesn't want to turn 30, eh?
2. And inability to remember what months they were born in. Maybe this is where the whole age issue comes from, because Rachel tells Gunther that her birthday is in May in "The One With Joey's New Girlfriend," yet later says she's an Aquarius in Season 7. The problem? If you're an Aquarius, you're born in January or February.
3. The locked room mystery. In Season 10's "The One With The Late Thanksgiving," Monica and Chandler lock the others out of their apartment to punish them for being late. Rachel unlocks the door after digging out an "old key" she still has from her time living there, but the chain's on and the (now iconic) heads-through-the-door moment happens. Unfortunately, that key shouldn't have worked because the locks to the apartment were changed in season 8, AFTER Rachel moved out, when Joey tells the superintendent there's a gas leak to make him break the door down. Oops.
4. Disappearing ink. During the Season 5 finale, Ross and Rachel get married with permanent ink all over their faces. The whole POINT of the episode is that this ink won't come off, right? And yet at the beginning of Season 6, when they're having breakfast, the ink has magically disappeared. How did they get it off? ANSWERS, WE NEED ANSWERS.
5. Ross also lied about his feelings toward ice cream. In season 7, he claims he doesn't like ice cream (blasphemy) because it's "too cold," but we see him happily tucking into a cone with Marcel the monkey and girlfriend Elizabeth on two separate occasions. This is why we have trust issues.
6. Rachel's and Chandler's memory loss. In the pilot, Rachel and Chandler have to be introduced, as if they've never seen each other before. This contradicts later plot lines, which show they met on at LEAST four separate occasions during high school/college—even making out in a flashback during season 10. It's unlikely they wouldn't recollect any of this, no?
7. Chandler's shock at Monica's weight loss. In actual fact, Chandler seems to regularly suffer from amnesia. Remember how shocked he is in "The One With The Prom Video?" Monica tells him the camera adds 10 lbs., he asks her "how many cameras were on you?" Aside from the obvious body-shaming, this doesn't make sense—Chandler regularly met Monica when they were both younger. She even "accidentally" cut off his toe when he visited the Gellers for Thanksgiving, after she'd heard him calling her "fat" the year before.
8. Does Phoebe know how to speak French or not? She has no idea what "sous chef" means in "The One With Rachel's Date," but appears to be pretty much fluent when she tries to teach Joey the language for an upcoming audition. C'est un mystère.
9. Chandler CAN cry. There's an entire episode insisting he can't—it's literally called "The One Where Chandler Can't Cry," and features the friends trying to garner some sort of reaction from his tear ducts, to no avail. HOWEVER, in an earlier season, the gang are seen talking about the time Phoebe made Chandler "cry like a baby," making the entire plot line of the episode pointless.
10. The change of venue. Remember when Monica decided that that museum was the perfect wedding venue and signed up on the waiting list? Then they called about a cancellation, but Chandler picked up the phone and freaked out before actually proposing? Well, they didn't actually end up getting married there. All that fuss and they pick another venue? GUYS C'MON.
11. The double declaration of love. Chandler's not the only one suffering from memory loss, apparently, because Monica seems to forget the moment he says he loves her for the very first time. Chandler originally drops the L-bomb when Monica's dancing about with a turkey on her head in "The One With All The Thanksgivings" (natch). "I can't believe it.. You love me!" she says from within the bird's carcass. However, in a later episode, "The One Where They All Find Out," Chandler tells the group he's in love with Monica again, while she's hiding in the bathroom, and she appears just as shocked as the first time round. Because she didn't expect him to tell everyone? Or because she's actually completely forgotten he's already said it? Who knows.