Continuity mistake: In the last few seconds of season 3, Ross closes the door immediately after he steps through it. However, in the opening seconds of season 4, we see this same event from the other side of the door. This time however, Ross doesn't close the door for at least 15-20 seconds after entering the room.
Continuity mistake: When Ross goes to Amanda's house for a date you see her two children sitting on the couch. The little boy with blonde hair is sitting down, when it cuts to Ross and Amanda talking the boy is standing up on the couch, the next shot he's sat down as Amanda walks out the door.
Other mistake: When Emily first walks in wearing her wedding dress, there is a distinctive difference between that dress and the one Monica picked up from the store a few episodes earlier and tried on (yes, more than that she is also wearing a jacket). There is a band of material at the top of the dress when Emily is wearing it that was not there when Monica wore it.
Other mistake: When Ross first goes out to play rugby and goes into the scrum, just after Emily talks to the rest of the players, he jumps into the middle of the scrum where the ball should be. Instead, in the next shot, the rugby ball is to the far left of the shot, not in the actual scrum.
Visible crew/equipment: When Phoebe and Rick are sat on the massage table, keep an eye behind Phoebe's head when the camera is on her, particularly after Rick says, "I can always find another masseuse." You can see the shadow of the boom mic moving.LittleWildman
Continuity mistake: In the scene where Joey is showing kids around the museum, there's a little boy who corrects Joey about the existence of the Mastodon. The kid's arm is by his side at the beginning of the scene. When he begins saying, "Isn't the Mastodon from the Pliocene Epoch?" his left arm is suddenly up and his thumb pointing at the Mastodon. (00:07:05)