Sorry to hear that anon. There's a few ways a punch can go wrong. The first and simplest is by scuffing your knuckles across the target. Everyone does this all the time when they're first starting martial arts. I've lost count of how many scabs I formed and then tore open by scuffing my knuckles. Something helpful to keep in mind for this is that you want to strike with either the top 2 knuckles (index and middle finger), or the bottom 3 (pinky, ring, and middle). Those knuckles should hit the target and dig in, then come straight back to your face for defense, no sliding across the target.
The second way that a punch can go wrong is by closing your fingers over your thumb when you make a fist. NEVER DO THIS. It may not happen on the first punch, or the 100th punch, or the 1000th punch, but eventually you will break your own goddamm thumb by doing this. Finger bones and joints are fragile, and easy to break if a relatively small amount of force is applied in just the right way. And your thumb is NOT meant to have the full force of a max strength punch bending that knuckle joint, which is exactly what happens if your thumb is inside a closed fist; it's in the force load path. If your thumb is outside of a closed fist, then landing a punch doesn't transmit any force at all through the thumb.
The next way I can think of for a punch to go wrong is by not holding your wrist locked and steady enough. So basically, you connect with the target, but you hurt your wrist like hell in the process. There's probably a few others, but those are the ones that come to mind immediately.
As for my own feels:
>spent the past several months (on and off) working on punching form
>get to a stage I'm decently happy with
>spar with training partner
>he's much better and more experienced than me, but I land a few good hits
>coach sees us sparring
>"Anon, your footwork looks like shit. We're going to need to fix that."
Life is suffering.