Urrgh

In season 9, Phoebe says that's she's never had a long term relationship - not even a relationship that's lasted longer than a month. Also that she'd never lived with a boyfriend before. But in season 5 she moved in with Gary (granted, not for that long). And if the span between most episodes is a week, then she also dated him for 5-6 weeks

In 'TOW Mrs.Bing' (S1), Joey says that it's okay to kiss a friend's sister, but still he gets mad when Chandler kisses his sister in season 3. (Although this doesn't necessarily make it a mistake, since maybe Joey just doesn't apply this rule when it comes to his own family)

In 'TOW All the Thanksgivings' (S5) a flashback is shown of 1992 including Joey, Phoebe, Monica and Chandler. Only, in 'TOW the Flashback' (S3), Joey doesn't meet and move in with Chandler until 1993.

In 'TOW Ross Meets Elizabeth's Dad' Rachel says that she and Ross were married way longer than Ross and Emily. But in the fifth season New Year's-episode, it is mentioned that Ross's divorce with Emily isn't final yet, whereas Ross's marriage to Rachel ended 5 episodes before that season's New Year's-episode (that for that matter even took place a few weeks before New Year's). And since Chandler and Monica's 'hook-up' anniversary is the date before Ross and Emily's wedding (making their Vegas trip take place almost exactly the same date as the 'anniversary' of Ross's second wedding)

In 'TOW Everybody Finds Out' Phoebe states that she and Chandler are about to kiss eachother for the first time - but they kissed twice just 12 episodes before!

And a Goof:
In 'TOA Ross says Rachel', the buildings in the window behind Phoebe sway - revealing the fact that it's just a poster