It happens...
In my opinion there is no excuse for "intentionally" hurting your partner. But in any type of fight training there is one thing that is inevitable....YOU ARE GOING TO GET HIT. If you don't then either you are the best ever or more likely you're not really doing anything.
When I started my training back in the day we never used pads, gloves or anything so we had to learn from very early on control. But trust me when I tell you we took more than our fair share of bumps and bruises. I've caught a roundhouse to the nose, been punched and kicked countless times, hit with staffs (except for in the face thankfully) and never once did it deter me or my training brothers (and sisters) from continuing. Hell I'll never forget the first time I sparred with a girl. I was so leary of being too rough and she caught me dead on in the face - appologized and we moved on and I learned really fast not to underestimate anyone...and that I needed to get the hell out of the way of that hit.
Recently I took a hit right to the nose and started bleeding like a fool. The key thing is this...NONE OF MY PARTNERS EVERY INTENDED TO HURT ME or I THEM. Look you can have great control but the fact remains that you are not in 100% control of the situation. There are variables that you can not account for. If I feint and he tried to anticipate a move and my feint actually hit him then...oops. I wasn't going to smash him anyway so odds are it may sting but it's not going to create any lasting damage. I've tried to evade punches and evaded myself right into something else before...again it just happens and it's part of the training. There is also some benefit to getting hit...training yourself to get over that gunshyness or fear of being hit. You can't do that if everytime you get hit you walk away, get angry or stop. WORK THROUGH IT (within reason of course).
To quit because you accidentally hurt someone does neither yourself nor your partner any good. All you've learned is how to quit and not how to control yourself better next time and he never gets to learn what he should or could do going forward. So basically you may as well have just walked up to him and punched him in the face and said have a nice day.
If you don't mean to do it and you are "conscious" of your actions then your partner knows (or should know) that it was unintentional. I don't even like to call it an accident because honestly you were intent on hitting them just didn't mean to connect with the amount of force you did. Accident means I had no intent to ever come close to doing what I did. If I cut your ear off with a sword or stab you with a spear...THAT'S AN ACCIDENT (I would hope).
Control yourself...take the hit...suck it up and keep training.
"To know you don't know is best.
Not to know you don't know is a flaw.
Therefore, the Sage's not being flawed
Stems from his recognizing a flaw as a flaw.
Therefore, he is flawless."