Series 3, Episode 17
The friends are caught in the middle of the whole Ross/Rachel split and trying not to take sides. They leave Ross behind in favour of a ski-ing trip but they run out of gas en route (just after they get themselves locked out of the car and have a debate about which of the three girls has the biggest boobs so they can use the underwire from their bra to get back in!). When Ross turns up to help them out it creates a heck of an atmosphere with Rachel. Ross finds solace (or tries to) in Carol, but she feels put out by it.
Oooo there is simply one word to describe this episode: uncomfortable!