Mystery man flexes in proposal photobomb going viral

A photobomb of an engagement portrait is going viral thanks to a mystery man who flexed in the foreground just as the big question was being popped.

The photographer captured a series of photos as a man was proposing to his soon-to-be fiance in a Chicago park.

As he's asking the question, a man in the foreground walks into the frame, notices the photographer and appears to be flexing or caught in mid-jog - though it's hard to tell if he did so intentionally.

Nobody knows who the man is yet walking through the pictures.

The newly engaged couple has a good sense of humor about it, saying they'd like to meet the mystery man - and he should photobomb every one of their life events going forward.

They even went as far to extend him an invite to their wedding next year.