The One With Phoebe's Husband - S2-E4
Character mistake: When everyone hears that Chandler has a third nipple, even Ross is surprised. He's known Chandler for many years - and they were college roommates - so Ross would certainly know this fact.
Suggested correction: A third nipple is usually much smaller than the other nipples, several inches away and looks more like a mole. For example, Mark Wahlberg has a third nipple that looks like a mole. It is likely if Ross saw Chandler without a shirt on and saw anything, he assumed it was a mole and Chandler did not tell Ross what it was. Chandler likely mentioned it to Joey.