I don't smack my two and I can only recall being smacked a few times as a child.
My hubby was regularly beaten by his father and is very anti smacking anyway. My kiddiwinks are still quite young so I find the naughty step works(not for hubby though) but who knows how I'll react to a situation when they are older. I'd like to say I'd never smack them but I just don't know.
You can't beat yourself up for one smack - I bet there have been plenty of other times when your son has tried your patience or been naughty and you haven't smacked him.
As parents we can only do our best children aren't born with a manual