In the 14th episode of Season 3, titled “The One With Phoebe’s Ex-Partner,” Chandler makes plans with a woman named Ginger whom he met while waiting for the bathroom at Central Perk. As Ginger leaves the coffeehouse, she spots Joey, who attempts to avoid her by hiding behind a coat rack (David Schwimmer nearly breaks character as he watches Joey hide). Later, when Monica and Pheobe mention that Chandler has a date with Ginger, Joey freaks out … and then makes a horrible confession.
Years ago, Joey was in a new relationship with Ginger when the two decided to enjoy a weekend at her father’s cabin, with Ginger’s dog in tow. In the middle of the night, Joey woke to find the fire dying down in the fireplace, so he tosses another log on the fire … or at least what he thought was a log. Jumping to the obvious conclusion, Phoebe and Monica assume Joey accidentally threw Pepper the dog onto the fire, to which he replies, “I wish.” No, it wasn’t Pepper that Joey used to boost the flames, it was Ginger’s prosthetic leg.
At this point in the story, Joey, while guilty of general stupidity, didn’t cross the line into the unforgivable. That came when he realized his error, and instead of waking Ginger to apologize, he instead ran away, leaving her behind with one leg, and never talked to her again. The friends have made some poor relationship decisions over the years, but it’s tough to imagine one worse than burning a woman’s artificial limb and then abandoning her in a cabin in the woods.