'The One Where Joey Bursts Himself'.
'Friends' isn't really known for its stunts but it turns out it was a sometimes hazardous place to work, especially for one Matt LeBlanc.
The episode in particular where it happened was 'The One Where No One's Ready', arguably one of the best if not the best episode of the series, and it was the fight between Joey and Chandler over the chair that resulted in LeBlanc's arm looking like it was coming out his ass (his words).
And that's not even the strangest part because then Dr. Mark Greene from 'ER' showed up.
But now you know that those episodes where Joey's arm is in a sling because he was "jumping on the bed" was actually because he took a beating all for our entertainment.
A true hero.