In a dialogue between Joey and Chandler, Joey says that he knows all about Kip (Chandler's ex-room-mate). But in the later season, when Rachel says, she's going to be the next Kip, Joey doesn't know who Kip is.
Even though in the previous episode the apartment numbers were 19 and 20, they're back to 4 and 5 in this episode, but later returned to 19 and 20.
At around 16 minutes when Phoebe and Rachel are talking, Rachel pulls at the roll of paper towels, causing a long row of them to follow her and tears the first one, very shortly after she has a single untorn sheet, shortly after it's attached to the rest. Very shortly after this, the unrolled paper towels roll back up and then cut to being unrolled again.
Chandler and Joey buy their foosball table with red and blue foosball players. In later seasons, the players are yellow and black. It isn't mentioned that Joey bought a new foosball table, just that Joey had kicked it and broke one of the legs and all of a sudden, in The One with Monica's Thunder (2000), the new table appears...
At the end of this episode Monica, Chandler and Joey are playing Joey and Chandler's new table football game at 02:30 in the morning, and Monica apparently scores. However, Monica is obviously controlling the red team but the team that is actually seen scoring is the blue team.
Monica makes meat lasagnas when her aunt actually wanted vegetarian lasagna. The entire episode she is trying to get rid of the dozen lasagnas. Toward the end of the show, Phoebe eats the lasagna. Phoebe is a vegetarian. Response: Monica says only every third layer has meat. Phoebe could be eating the non-meat layers. Also she is not a strict vegetarian. In The One with the Fake Party (1998), she knowingly eats meat.
When Rachel is drinking from the thermos, she drinks long enough to necessitate several swallows, but she is clearly not swallowing.
(at around 45 mins) When Chandler and Joey enter their apartment in the top left corner studio lights can be seen hanging down into shot.
In Chandler and Joey's apartment, stage lights are reflected on the television screen.