In the episode after "The One With The Proposal 2/2" the gang are getting ready to go to a party to celebrate the engagement and Monica is wearing a red dress. She then goes into the bedroom, talks to Chandler then comes out wearing a black dress. Freaky, huh?
First off, it's the TOW Monica's Thunder, Episode 1 of Season 7.
Second, it's not a slip-up.
The gang decides to go have drinks to celebrate, so they all go off to get dressed.
When Monica and Chandler go into their room, Chandler decides he wants to do it before getting dressed up. So they get in bed together.
Then we see a scene of Joey and Rachel in their apartment talking.
Then there's a transition, usually meaning that an unknown amount of time has passed. They do that between scenes a lot.
We see Chandler and Monica coming out of the bedroom, dressed up, talking about how Chandler wasn't able to do it(if you know what I mean).
So, that is not a slip-up.
Obviously, after Chandler couldn't make it happen, they got dressed and ready to go.
no no no. thats all wrong. when they all decided to go to the plaza to drink champagne, monica was wearing the same outfit she was wearing when chandler proposed...a black shirt, red skirt, and white jacket. after they all decided to get ready monica was wearing a red dress. the black dress you see her in later is a slip.