Corrected entry: When Ross is climbing up the fire escape and reading Rachel's 'pro list' to her, it is raining really badly. However, when the other four friends walk into the apartment, they are all dry.
Corrected entry: When Ross shows Rachel Julie's 'Con' list, he appears to turn over the paper and show Rachel the other side - but his deskjet-type printer would not print on both sides without him having reinserted the paper, which he doesn't.
Corrected entry: In TOW, the list Monica says that she is allergic to cat hair. In TOW Ross Finds Out we find out that Monica used to have a cat named Fluffy Meowington. To me that sounds like an animal with a lot of cat hair.
Correction: Of course they are dry. If you look closer, you can see they are all holding umbrellas.