When Chandler and Phoebe get to the front of the set for the ceremony, we see snow on the back of Chandler's jacket from previous takes where he sat in a snow-covered chair.
When the ice sculpture is delivered to Joey's apartment, Monica says it must have been sent to the billing address, not the shipping address. Phoebe doesn't live in that apartment, so wouldn't be using it as her billing address.
Monica says "Alright, people, it's zero hour," which is often interpreted as midnight. Seconds later, she schedules Chandler for sex at 2300 hours, which is 11pm, an hour earlier. And it wasn't 2300 the next day because at the end of the episode, Chandler and Joey had just returned from the wedding ceremony, and Chandler says, "It's 2300 hours, and I'm about to have the most organized sex anyone's ever had." However, in common vernacular, "zero hour" simply means the time at which a planned action is to take place (in this case, the wedding ceremony), in the same way people will refer to "D-Day" without actually referring to the Allied invasion of Normandy.
Chandler tries to convince Rachel to choose him to be one of the groomsmen by telling her how he used to be picked last in gym, a problem she did not identify with. However, throughout the series, and especially in The One with the Football (1996), she complains that she always gets picked last in sports.
When Phoebe tells Monica she wants her help, the headset that Monica says she missed so much is not the same as the headset she wore during the rehearsal.
In the HD release you can see a hole in the wall of Joey's apartment to the right side of your screen. Just after Ross and Chandler come in to ask Rachel whom she picked to be Mike's groomsman, you can see someone in the hallway behind Joey's apartment wall wearing a red sweater. Shortly after this, Mike enters the apartment wearing a red sweater. He was obviously waiting in the hall for his cue.
As Ross says "You're not going to be in the wedding, either" to Rachel, the boom mic shadow is visible on the wall behind them.
In this episode, Monica says that she has never been fired before, but in The One with Five Steaks and an Eggplant (1995) she gets fired from her job for accepting a kickback of five steaks from the meat vendor.