Justin Bieber Rocking SKIMS Thanks to Billboard Mashup

Forget about where Justin Bieber gets his peaches -- the better question is where he shaves his legs ... at least based on this hilarious billboard swap that has him slipping into Kim Kardashian's underwear line.

Check out this giant tower-sized Balenciaga ad on a Sunset Strip building with Bieber as its leather-jacketed star -- but as you can see, he's lost his lower half somewhere along the way.

Instead of JB's lower abdomen and legs, you see a different set of body parts ... seemingly belonging to a woman rocking SKIMS, Kim's shapewear company.

It's pretty hilarious -- it looks almost intentional due to how seamlessly JB's top mixes with the SKIMS bottom. Even the SKIMS model's arms appear to line up almost perfectly with the Biebs'.

Of course, this had to have been a massive mistake. We can't see a crew working on it in the pic ... but we're guessing they were in the middle of swapping from one ad to the other.