In "The One with Joey's New Brain" (S7 E15), Phoebe tells the man who left his cell phone in Central Perk that she lives at 5 Morton Street, Apartment 14. However, when Monica knocks on Phoebe's door, it clearly shows the number 16.
When the surprise shower guests come out of their hiding places, there is no way that Monica would not have at least seen the woman behind the couch and the woman behind the chair in the lower right corner of the set based upon where she is standing during the dialogue that precedes this moment.