The baffles within the muffler (turbo style) would face the wrong way and it would be much more restrictive than the correct way, it could create a bunch of back pressure but would probably run, expect exhaust leaks I would say. My guess is it would sound different as well, Now the question, why do you ask?? But it certainly could be done surely by accident, just being sarcastic I hope whoever did this didn't build the motor as well!