Ross insists to Monica and Phoebe that he gave Mona "the only key" to his apartment. In previous stories, Monica and Rachel would routinely go over to Ross's place in his absence to borrow cooking ingredients (that is also how they accidentally deleted a phone message from his ex-wife Emily). Phoebe also sometimes massaged her clients at Ross's place while he was at work.
At the beginning when Ross and Mona are looking at their pictures from ice skating, Mona hands Ross the picture she thinks they should use for their holiday card. When the shot cuts back and forth from Mona to Ross, the number of pictures he is holding switches between one and two.
After Ross gives away his key to Mona, he claims that was the only one and was "homeless." In every apartment building there is a super or maintenance person who has spare sets of keys.
In "TOW Phoebe's Cookies", Chandler had announced that they wouldn't be having an engagement party. However, when talking to Chandler after talking to Doug, Monica says that Doug wasn't invited to his party due to his behavior at their supposed engagement party.
In the episode TOW Phoebe's cookies, when asked about engagement presents, Chandler announced they would not be having an engagement party. However when speaking with Chandler after speaking with Ross, Monica states Doug was not invited to their wedding due to his behavior at this supposed engagement party.
In the episode TOW Phoebe's cookies, when asked about engagement presents, Chandler announced they would not be having an engagement party. However when speaking with Chandler after speaking with Doug, Monica states Doug was not invited to their wedding due to his behavior at this supposed engagement party.
It is now at least a week into December -- Rachel conceived in April (known because Monica & Chandler's wedding was booked for May) -- therefore, 8 months have passed. Rachel should be due sometime in January and quite large by now. She maintains in dialogue with Phoebe that she is only "4 months", a total chronological inconsistency.