Rachel's boss is called Mr. Zelner however when he went to her office, she called him Mr. Zelder. This was not corrected by himself.
All through the show, Phoebe's bike had a bell on the right handlebar and a mirror on the left. In the final scene, the two accessories have swapped sides.
When Ross is trying to teach Phoebe how to ride a bike and she is sitting on the swings, her scarf goes from part of it hanging over her left shoulder in the close up, to hanging over her right shoulder in the long shot and then switches back again.
Ben mouths Ross's line as he is saying, "One dad, two moms."
When Phoebe falls off the bike in the park after Ross lets go, you can see that she is actually standing beside the bike and then drops it, instead of falling off from a sitting position.