Phoebe tells Monica that she slept with Jason Hurley and Monica is shocked. But in TOW George Stephanopoulos, this is already mentioned and Monica is less shocked then.
When Rachel is reading question number 29 in the book, she has a ball pen in her right hand. The next shot the pen disappears from her hand and she picks it up from the table to write.
When Rachel is talking to Ross in the coffee house about taking her wind, the book keeps switching from front to back in her hands between shots.
When Joey does the script reading, he misreads "50 bucks" as "so bucks". In an actual script, "50" would be written as "fifty".
In the beginning when Eddie leaves Chandler's bedroom, there's a wall with a poster and a coat rack directly outside Chandler's door. But Chandler's bedroom opens directly into the living room so there shouldn't be a random wall there.