When Ross is looking out the window saying "No, no, what are you doing? Get off my sister." There is a reflection of the windows opposite, in the window that Ross is looking out of, but no-one can be seen in the reflection, indicating that nothing is happening for Ross to shout at.
While Chandler says "Nothing, I said, I said you're so great and then I just stopped talking" a piece of debris floats down from the lights just behind him.
At around 2:22 when Phoebe says "Your breasts look kinda small", you can clearly see that her mouth is not in sync with the audio for "look kinda small" and that it was dubbed over.
There's a mannequin next to the window behind Chandler and Phoebe, while she is trying to flirt with him. It looks a little bit like Joey, only the arms of the shirt are empty and it's not moving, which indicates that it must be a mannequin. The face is blurred and there is no hint in all of the episodes, that Joey, Chandler and Ross keep a mannequin in their apartment. Don't know if it was intentional, or a crew member was moving around the apartment trying to fix something and got stuck in the background, while the scene was being shot.