When Phoebe is singing her song about how when she first met Chandler she thought he was gay, the door to the guest room is open. Then Monica says something to her and in the following shot, the guest room door behind Phoebe is closed.
Monica says that Rachel is jealous of her because she's getting married first. But in ep 24 of season 5 "The One in Vegas: Part 2" Rachel and Ross got married in Vegas.
Even though this episode is supposed to take place the day after The One with the Proposal: Part 2 (2000), that episode was filmed the previous season and Chandler's hair is very different and he is significantly thinner.
When Chandler is in the recliner at Joey's, Joey asks if he can pass for nineteen. Chandler replies he cannot pass for nineteen, but "nineteen" is clearly dubbed in as it does not match what Chandler is saying.
Right after Joey comes out his room in the teenager outfit, there is a shadow of a crew member moving from right to left.