In the movies, everyone from lab scientists to sassy sidekicks throw punches all the time, with no ill effects to anyone but the recipient, who tends to be knocked out without a hitch. The worst case scenario is that the punch has no effect and the opponent will simply be amused, like that giant Nazi Indiana Jones couldn't hurt without a plane propeller. Right?
Of course it's not right. Have you read this site before? Is it ever "right" when we ask like that?
Here's the problem -- the hand is a pretty delicate thing. A fight-worthy fist is a lot more than just a bunched-up hand -- developing your curled fingers into a punching tool takes years of training. Even real boxers get it wrong often enough that the most common injury caused by punching failure is known as boxer's fracture.
So, what's the worst that could happen if we get it wrong? Well ... everything, really. There are at least as many ways to break your own fist with your opponent's body as the other way around. Say you align your fingers ever so slightly wrong. Too bad, they are now broken. Hit the target with the wrong knuckle? Enjoy the dislocation of said knuckle. Get the angle wrong? Congratulations, you now have a broken wrist.