When Tag brings Rachel a plant to liven up her office, he is surprised to see there is already a plant near her desk; it's sitting on the ground to the right of the door, and parts of it reach up as high as the painting hanging on the wall. But in a shot of Tag as he's leaving her office, that same spot is visible, but no plant can be seen.
When Joey drops the phone after Mac And Cheese is dropped it bounces out the chair and falls on the floor.
At around 16:58, Joey tells Rachel and Phoebe he was expected to audition for a role. Phoebe's response includes the word "madness". Right after she says it, the camera cuts to Joey, but if you look at Phoebe, you can see she is saying "madness" again, although no sound is heard from her.
When spilling secrets, it's revealed that Ross slept with the cleaning lady in college, although earlier in the series it was established that he only had slept with Carol up to that point.