Friends mistake: The one at the beach/The one with the jellyfish 

That's funny, I was trying to figure out what was the mistake and I didn't pick up that he didn't close the door, but I immediately realized that the rooms are different :) When he opens the door in the first scene, there is a wall right next to the door. But next time there is some space and a drawer next to the door.

Great pickup! But there's also a continuity error. Look at Ross' hair in the "One at the beach" episode, his hair is short, spiky and has no sideburns. For the first episode of Season 4 which is "the one with the jellyfish", the shot of Ross outside in the hallway between the two doors is the same shot reused from the last episode from season 3. However, when Ross enters the room at 0:40, his hair is noticeably longer, it is full of product, it's combed and his sideburns are much longer in length. Just compare the two shots at 0:37 and 0:40!! So what, Ross' hair suddenly grew in 2 seconds???

Rachel was kind of evil for doing that. Pretending to help her by putting ointment on her bald head then turning around and kissing Ross.

Also the way he says hi is more Ross-y in the jellyfish episode

Nice nitpick