Here are some examples:
1. Rachel and Ross's length of dating.
In season 3, Rachel says to Ross that he should trust her, and that they've been dating for almost a year, but it seems that they've only been dating for six months; because they started dating towards the end of season 2, and it's only halfway through season 3.
2. Now Rachel and Ross dated for two years?
In a later season, Rachel says that she and Ross dated for two years, but they only dated for one year excluding a long break-up, and one night of being back together.
3. Monica's age.
In season 1, Monica says she's 26, but in season 6 she turns 30. She should be turning 31 or 32.
4. Chandler and Monica's wedding.
In season 7, in an episode where Monica is in a bridal shop with Phoebe and Rachel, she states that she is getting married in May. But did they? If this is when Rachel is six weeks pregnant, and in season 8 in the episode where the baby is late in arrival, Rachel says it's 100 degrees out. If she was six weeks pregnant in May, then she should be having her baby in December, and we know it's not 100 degrees in December in Manhattan.
5. Emma's birthday and Chandler and Monica's infertility.
Chandler and Monica started trying to conceive a baby on the day Emma was born. In a later episode, Monica says to Chandler that she thinks they should take a fertility test because they've been trying to have a baby "for over a year now." But how can this be, when Emma's 1st birthday isn't for SEVERAL more episodes? In fact, her 1st birthday is not until the middle of season 10.
6. The main characters' birthdays.
The mentioning of the friends' birthdays sometimes do not add up when stated other times, or shown.
What do you think?