After the "boyfriend bonfire" gets out of control and the fireman are over, Monica is standing beside Rachel and we can see her holding a piece of something in her hand. This can be identified as the partly burned picture she threw into the fire. However moments later, one of the fireman pulls this photo out of the garbage bin.
At the beginning of the season, Monica and Rachel's apartment number is 5, and Chandler and Joey's is 4. Now Monica and Rachel's apartment number is 20, while Chandler and Joey's is 19.
When the woman in the red jumper walks past Ross and Carol in the restaurant, she enters the shot just before the table and walks up to the halfway point, we then see the woman from the back and she is at the end of the table and walks past it.
Phoebe mentions her friend named Abby who shaves her head, but in later episodes when her bald friend starts dating Ross, her name is Bonnie. However, Phoebe could easily have more than one female friend who shaved their heads, which would fit with her quirky personality.
When Ross hugs Chandler, the egg cracking is heard, yet there is no yoke on Chandler's sweater or eggshell. However, Ross is wearing a suit with a jacket, and the egg yolk would not be visible if the egg was inside his pocket.
In The One with the Dozen Lasagnas (1995), Carol mentions that Susan is a vegetarian but while at the Japanese restaurant, you can clearly see a lobster tail on Susan's plate. However, some vegetarians eat fish. Susan may simply not eat meat.