When Rachel is pulling the pictures off the wall that are covering the holes, there is a 'glitch' showing that something was edited out as Rachel is pulling off the 4th picture from the wall. It is as if Rachel took more than one attempt to peel of that picture.
During the rugby game the number of chopsticks in Phoebe's hair changes between one and two.
When Ross is playing Rugby with Emily's friends, one of them yells for Ross to get into the scrum, while Ross is standing out of it while the others are in it. The rules of Rugby dictate that all players that are involved in the scrum (including Ross) must be present at the start of it. Because Ross wasn't present at the start of the scrum, they would have to start it again, rather than them call him midway through the scrum.
For travel to Yemen, Chandler would have needed a visa. Issued by the Yemeni consulate. This would have been checked by the airline before boarding.
There are numerous errors in the rugby game. Most notably, the game does not begin with a scrum, and there are 15 players per side, as opposed to the 5-a-side depicted. The goal posts and try lines are 100 metres apart - rugby cannot be played in a small park.
Emily states loftily, "Well I mean, you're American to start with. You don't even have rugby here." In the United States, Rugby is indeed played on a professional, semi-professional, collegian, high school and club level in addition to pick up games.
To be fair, Emily may not have known that rugby is played in the US - she was probably making an assumption. After all, Ross didn't know what rugby was either until Joey showed him a video of the sport.
When trying to remind Joey of Red Ross, Ross mentions the time they were waiting in line to see Dances with Wolves (1990). The movie came out in 1990, three years before Joey joined the group.
Ross says that this is the first time he's made a grown man cry then remembers he made his father cry when he was four years old when he washed the Porsche with rocks. However, Jack told Richard at his birthday party that he bought the car when he turned 50. Based on Ross' comments, Jack would be in his 90's if Ross was four in 1971.