In the ninth season, episode two, Ross visits the hospital after injuring his hand trying to punch Joey, and tells him his birthday is 18 October.
In an earlier episode, he tells Gunther in season seven that his birthday is in December.
As fans also point out in season three he says he is 29. In the fourth season, he also says he is 29. In the fifth season, he talks about being divorced twice before turning 30.
He has stayed 29 throughout his relationship with Rachel and their breakup, then his meeting with, engagement to, and eventual wedding day with Emily. Then for some time further during the aftermath of the botched ceremony. And during three Thanksgivings.
Rachel also tells Gunther in the seventh season that her birthday is in May, having previously said she was an Aquarius (meaning a January or February birthday).
She also becomes the last member of the group to turn 30 in a seventh season episode, which starts flashbacks for the other members of the group.
Joey has previously been described as the youngest member of the group, aged 25 at a time when Rachel and Monica were 26, having been in the same school year.
3. Ross' false claim that Carol was his only partner at the time of their divorce
In the first season, Ross tells the guys that his ex-wife Carol is the only woman he has slept with at that point.
He maintains this story for a few seasons, including with Rachel in their relationship - making her the second woman he'd ever been with.
In the seventh season, Ross accuses Chandler of sleeping with the cleaning lady in college. Chandler points out that, actually, Ross was the one who did that.
This is relatively consistent in that he'd supposedly forgotten it was him, but it's farcical that he would forget what was supposedly a large emotional fulcrum for his character.
So there you have it, a number of inconsistencies to grate your teeth at.
Oh, and that Bruce Willis shows up as Rachel's date to meet Die Hard fans Chandler, Ross, and Joey and they don't say a damn word about the likeness.