At Central Perk Phoebe asks a sip from Joey's cup and a bite from his muffin and Joey agrees without any hesitation, but in The One with the Birth Mother (2004) it's very clear that Joey doesn't share food and is very unconditional about it.
When Rachel goes back to her office with Emma and Ross, her name on the door is spelled Rachel Greene - as it is on the RSVP for Ross's wedding to Emily - but her character's name is always listed elsewhere as Green.
When Ross agrees to take care of Emma for the day, he leans forward and kisses her but the pink bow sticks to his forehead covering his eyebrows too. But in the next shot, the bow is attached to his mid-forehead region instead .
When Phoebe and Mike are discussing that giving away the rats babies it brings back memories of giving away the triplets of her brother the box is on the table. After the cut the box is on her knees again.
During the argument with Gavin, Rachel makes a big fuss over the fact that she has worked the job at Ralph Lauren for five years. This cannot be true, considering this episode was set in January of 2003 and Rachel got the job at Ralph Lauren in March of 1999 - meaning she hadn't even been there four years yet.