Factual error: Before Ross goes out to play the second half of the rugby game, Emily gives him some tips on how to hurt the other guys. She singles out one of the guys, saying he doesn't wear a cup. These guys are all English players who love rugby. Rugby players would NEVER wear a cup for rugby is more of a grinding sport than hit and miss. These guys would be crazy to wear a cup.
Factual error: In season 4 generally, Phoebe has the shortest pregnancy ever. She gets pregnant as the boys win the girls' apartment. In the next episode Ross meets Emily, after 6 weeks he gets engaged to Emily and they then choose to get married 4 weeks later. Phoebe would only be 10-12 weeks pregnant by the time they fly to London, not in her third trimester.
Factual error: Rugby starts with a kickoff and not with a scrum, only the forwards on the team participate in the scrum not the whole team, and there is no scrum half to retrieve the ball from the scrum. The scrum took far too long. There is no structure to the scrum like there should be (props, locks, flankers etc). He doesn't seem to have a team and for some reason everyone just jumps on him, there is no line with one team and another it just seems like a free for all which is nothing like rugby, rugby halves are ended when the ball goes out of play which does not happen here - he just decides to end it for no real reason. Several fouls are committed in the ruck such as coming in from the sides, diving off their feet and holding onto the ball which aren't being penalised. You're not allowed to kick people or harm them intentionally in rugby, there are no cups in rugby but there are mouth guards, which no one is wearing.
Factual error: The guys are gone for a single night, watching the Knicks game. (You can tell this because they talk about how "we can go see the game tonight," and when they get back everyone's wearing the same clothes as that conversation.) When they get back, both apartments are completely switched back: the large, heavy furniture is replaced, the posters are re-hung, etc. Even if they were gone from, say, 6 p.m. to midnight or so, there is no way that Rachel and Monica would be able to make that transition in that kind of time.