When Joey spots the hot girl in Ross's building, he is wearing a dark fleece pullover. When he goes to knock on Ross's door for the first time, he is wearing a maroon button-up shirt. Then, when Ross opens the door, Joey is in the pullover again.
Every time Joey ends up at Ross's apartment when chasing after the hot girl, he has no idea that he is at the wrong door. Maybe that's because when Ross moved in "3B" was clearly screwed onto the door but apparently now has been removed with no trace.
When Joey ends up at Ross's apartment for the second time, we see that it is the last apartment on the left of the hall. Joey then goes back down the hall and knocks on the door before Ross's, and then on the door before that. When he runs away from that door he heads to his right, back down the corridor towards Ross which leads to a dead end, but somehow turns into a crossroad.
When Rachel returns from her 1. interview at Ralph Lauren's she takes out a bottle of water out of the fridge. In the next shot she is empty-handed and there are only Chandler's and Joey's bottles on the table. Rachel's bottle reappears for a second on the table when Chandler starts saying, "Well, you could have tried not kissing him," and then vanishes again.
In the episode "The One With The Girl Who Hits Joey," (Season 5, Episode 15) showing Ross in his new apartment, the walls were painted blue. In this episode, they are purple.
When Ross is in his apartment, doing his "watching TV" bit, he is sitting on the couch. However, when you see the reflection of his living room in the TV set, the couch is unoccupied.
After Joey asks the young girl if her sister etc is a hot girl and she yells, "Daddy!" and he runs away, the shadow of the boom mike is visible on the door when he shouts, "Oh man!"