The One With The Football - S3-E9
Corrected entry: When Monica and Ross are discussing why they are not allowed to play football, Monica states in the story that they were both 6. This would imply they are either twins or at least the same age. However, we see that Ross is in fact older than Monica. Also, later in the scene Monica and Ross say the football incident occurred 12 years ago. This would mean that they are both currently 18 years old which is false. In one episode earlier in the season Monica states that she is 28 years old. (00:02:20 - 00:04:10)
Correction: She doesn't say they were both six, she says the event happened in "Gellerbowl 6", ie. the 6th year of the "Gellerbowl".