I have only once accidentally cum during a spanking (many times it was done on purpose) with my wife.
The one accidental time was when she arranged for her friend to come over and witness my spanking. I didn't know it was arranged at the time but found out later that her friend had a spanking kink, and after my wife and I talked about how exciting it would be if someone else were to see our spankings, she set it up.
I was naked with my knees on the floor and upper body on the couch, legs spread, EVERYTHING very exposed, getting a spanking ( we do alot of play "punishment" spankings). When her friend knocks on the door. She tells me she is going to let her in and watch. I didn't resist much, because the idea while kind of scary, was really hot.
She tells me to stay in position and goes to the door, come back with her friend.
She walks in and is greeted by me bent over naked, dick and balls hanging down, hole exposed, everything. I look back at her, embarrassingly say hello, and bury my face in the couch.
She makes some comment about a naughty boy or something. She comes over and sits down and my wife resumes spanking. Between the spanking, exposure, and her friend there, I get an erection. Wife and friend comment on it.
As she's spanking, the movement is making my penis occasionally rub on the couch. The friction of that, plus the whole situation is too much. Within a few minutes, I feel it coming. I tell them "oh no, uhhh, oh shit" I push my butt out, point my dick down and start cumming all over the floor.
They both make some comments about it. Omg can't believe he did that. Look at the mess. Naughty boy. Etc.
She spanks me a few more times and then tells me to clean that up. So I have to go butt naked to the kitchen and get some stuff to clean it up. After it's clean the wife tells me to go to the bedroom and take a break.
She told me later that her and her friend talked and they both thought it was so hot and were so turned that they both masturbated right then and there (wish I could've seen that!)
We did on another occasion, all give each other spankings, but that's another story for another time.