Don't you wish you'd been a random bystander when THIS happened?!

Justin Bieber and Jimmy Fallon went undercover for the late-show host's "Musical Photobomb" challenge in New York's Central Park, wearing hilarious disguises and lipsyncing to Bieber's hit "What Do You Mean."

It was party of The Tonight Show's first ever broadcast from Central Park and while Justin was spotted in NYC on Thursday, ahead of the show's airing, the bit was taped in August.

Because if this had been taped yesterday, Beiber MAY have been a married man!