The magna-doodle in Joey and Rachel's apartment has the picture of a cat.
Phoebe claims that she's always wanted to live with a guy. She did, in fact, briefly move in with Gary in S5E21 "The One With The Ball"
At the end of the episode when Joey is getting food and beer from the fridge, the 6-pack (which is clearly coronas) says coron extras.
Mike's sweater is identical to the one Freddy Krueger wears in the Nightmare on Elm Street movies.