The voice Phoebe uses when speaking to Mike's parents is the Transatlantic accent, which was used by American actors in the 1930s and '40s.
In this episode it's revealed that Monica made a tape with Richard and Chandler says 'listen to the judgement from the porn star!' However in the fifth season Monica and Chandler get caught with a video camera set up, and in the third season Monica tells the group that she made a tape with Pete. Meaning she filmed herself during all three major relationships of the series.
Every time Ross or Rachel sings "Baby Got Back" to Emma, they always stop right before an inappropriate part in the song.
When Monica and Chandler are arguing Monica mentions that Chandler can't grow a mustache. However, in flashbacks Chandler is seen with a goatee. Meaning he can in fact grow a mustache.