When Monica and Chandler are trying to find out why Janine isn't coming over, the door to Janine's room is closed. When Monica starts walking towards Joey demanding to know why she isn't coming, her door is now open and you can actually see it opening a little on its own.
In the opening scene, Joey runs into Monica's apartment and tells everyone that he just kissed Janine. The kiss happened during the Christmas season and was shown at the end of the preceding episode. However, all of the Christmas decorations are now gone and everyone is in a completely different outfit, even though only about five minutes have passed between the two episodes.
Correction: The kiss happened the same night as the taping of dance parts for the New Year's Eve TV Show. It is mentioned in the previous episode (The one with the routine), that the taping of the dancing people happens well in advance, and they just edit them in the live show on NYE. So there wouldn't be any decorations for Christmas yet.
At about 18:37, when Joey and Janine are entering their apartment after the second time out with Monica and Chandler, the end of the set and a bit of the stage can be seen on the right-hand side of the shot.
After Janine avoids confrontation with Monica and leaves, we can see her starting to run twice in a terrible editing mistake.