Phoebe mentions to her half-brother that her birthday is February 16th. Yet in the episode "The One With Phoebe's Birthday Dinner", she states she made dinner reservations for her birthday on October 31st.
Ross enters Monica and Rachel's apartment with his laminated list of 5 celebrities he'd like to sleep with. He briefly holds the list towards the camera, which shows the names Uma Thurman; Isabella Rossellini; Elizabeth Hurley; Vanessa Williams; and Dorothy Hamill. He hands the list to Rachel who reads the names out loud. She reads Uma Thurman; Winona Ryder; Elizabeth Hurley; Michelle Pfeiffer; Dorothy Hamill.
In this episode Monica's toilet is on the wall opposite the door. In later shows (specifically The One With Monica and Chandler's wedding) the toilet is on the same wall as the sink, to the left of the door as we see it.
At the beginning when Chandler attempts to enter his bedroom through his newly-sawed-in-half door, he opens the top half and flips over the bottom half. Chandler's legs make contact with the bottom half of the door, however it remains closed as if held in place by something. In reality, since the door latch is on the top half of the door, the bottom half would freely swing and would have swung open as Chandler flipped over it.
Just after Joey puts the drill through the wall, Chandler comes out to shout at him; in the background is a section of the entertainment unit that appears to have been glued and clamped together, but the clamp running across the top is not putting any pressure on the glued joints and is, in fact, completely pointless.
Joey spends the majority of the episode making a new rack. But when the finished rack is shown one can clearly see the holes on the side that characterizes the flat-packed IKEA furniture.
After Joey almost gets Chandler with the drill, the audio looks like it's a second or two behind the visual when Chandler says "No you didn't get me. You get me, you kill me".
Joey supposedly has been building the unit for days, with Chandler right there, and only once they get the thing upright do they actually see how wide it is.
That's not possible. They'd have noticed how wide it is when it was still on the floor.
When Joey moves Monica's hamper to show her the color her floor tiles used to be she is shocked. It's hard to believe though that Monica, being the clean freak she is, hasn't moved the hamper to clean the floor before.