Ursula tells her fiancé that she wants to get drunk, but it is previously revealed that she does not drink when Joey dated her earlier in the series.
Chandler is dressed as a bunny. Monica says that she got him this costume because his favorite book growing up was "The Velveteen Rabbit". This was never mentioned when Chandler bought Kathy the first edition back in season four.
When Mona first walks into the apartment, Rachel says, "Hey Mona," but her mouth doesn't match the audio.
It is Halloween -- Rachel conceived in April (known because Monica & Chandler's wedding was booked for May) -- therefore, over 6 months have passed. Rachel should be due sometime in January and "showing" much more than she is now.