When Joey goes into Monica's fridge for a bunch of drinks for his audition he has a variety of drinks, including juice. When the camera goes back to him, he has a completely different set of drinks, all pop, no juice.
Phoebe and Ross get mugged in the night, but when they return back to the coffeehouse just afterwards, you can see sunlight in the background
When Joey is talking about his audition to Monica and Rachel (around minute 19), you can see Jennifer Aniston's stand-in next to Matt LeBlanc. She wasn't removed from the shot.
When Phoebe shows Ross the "Crap From the Street" box in coffeehouse the woman in a beige shirt is shown entering and sitting down twice.
When Ross and Phoebe are being mugged by Lowell, he hears Phoebe's name long before she recognizes him. Since they know each other, Lowell should've been able to greet her first and not be surprised to being acknowledged by Phoebe.
During a shot of the New York skyline the Met Life Building still had the old Pan Am building signage which was replaced in 1992 when the airline when bankrupt.
When Rachel enters Monica and Chandler's apartment to tell Joey that his agent called for the Leonard Hayes play audition, they discuss the monkey appearing at the audition. In the background Rachel is seen to have become shorter and wearing a different shirt. (Add-on: And becoming a different person. It's Aniston's stand-in)
With all his years of experience in acting, Joey should have known [long pause] in stage directions is not the name of a character.