The necklace that Judy Geller is wearing at the rehearsal dinner is the same one that Monica will wear at her wedding.
This is the only episode in which Jack and Judy Geller are seen at Central Perk. Judy also believes that Gunther is attractive, so much so that Jack tells Ross that Judy added him to "her list." This is a callback to The One with Frank Jr. (1996) when the gang made a list of celebrities they're allowed to sleep with.
When Joey first meets Richard Crosby (Gary Oldman), Joey says that Crosby just won an Oscar, though Crosby corrects him and points out he has lost three times. Oldman was nominated for an Oscar in 2012 for Tinker Tailor Soldier Spy (2011) but lost to Jean Dujardin, only to ultimately win an Oscar 16 years after the airing of this episode for his role as Winston Churchill in Darkest Hour (2017).
When freaking out about the wedding Chandler decides to see if he could prove that he and Monica are related. In real life Matthew Perry and Courteney Cox are distantly related on several different lines, but one is a Baldwin family from England.
Matt Leblanc and Gary Oldman both appeared in Lost in Space (1998).