I happened to catch one of the 10 zillion Friends reruns this evening, and they happened to show what appeared to be the pilot, but apparently wasn't. Central Perk hadn't formed yet -- instead, the hangout was a bar that turned into Central Perk.

Joey accidentally becomes Chandler's roommate, Ross and Phoebe unsuccessfully try to make out on the pool table, etc. And nothing in the episode involved Rachel leaving the altar. What episode was this?

That wasn't the pilot. That was "TOW the Flashback."