(TMZ) Michael Phelps broke something that wasn't a world record-- his rear end -- taking a nasty spill while crowd surfing in a Vegas nightclub this week ... and TMZ has the footage!Phelps -- who's STILL in party mode after the London Games -- hit up XS nightclub Monday night, and for some reason thought it would be a good idea to crowd surf inside an inflatable raft.
It was not ... because the 18-time gold medalist barely made it a few feet before toppling backwards onto his rear end.
Thankfully, we're told he was okay. Check it out.
