It was a single swat to the buttocks, done in full sight of everyone. There was other ass-spanking going on, between people who knew each other - done as a joke - so in context it was not so unusual. I would not have done it outside of that context, nor would I have done it if my inhibitions had not been lowered by alcohol; nor would I do it again even if they are.

Yes, she deserved it!

It was a mistake. Why? It exposed me to more risk than was worthwhile, and while I might have hoped that (aside from simple punishment) it would teach her the lesson that she ought to follow the Golden Rule, or at least should not pull the same tricks on guys, in retrospect it was unlikely to do so.

Other people (that I have talked to) seem to be divided on whether it was a good thing to do or not.