Even though the Friends never mentioned the terrorist attacks on the USA of 11 September 2001, in the opening scene of Season 8's "The One Where Chandler Takes a Bath", Joey is wearing a t-shirt with the emblem for the FDNY with the name Capt Billy Burke. Captain Billy Burke (William F Burke Jr) was a NYC Firefighter who lost his life on 11 September 2001 at the World Trade Center.
Matthew Perry received his only Emmy nomination of the series for this episode.
While discussing possible names for the baby, Ross suggests the name Helen which Rachel vetoes. In the second episode of season one, "The One with the Sonogram at the End", Susan suggested the name Helen for Ross and Carol's baby (before finding out that the baby is a boy who they later name Ben) and Ross replies,"Helen Geller? I don't think so."
When Ross reveals the sex of his and Rachel's baby, it could be considered an unintentional payback, since Rachel was the one who accidentally revealed Ben's sex to him back in season one.
When Monica draws a bath for Chandler and says she has a surprise for him. He asks if it's "sex on the balcony" and Monica says "no" and seems like that would never happen. However, in season 2 episode 4, in the opening scenes several secrets are revealed including that Monica had sex with Fun Bobby on the balcony.