Doc and I have a safe word, get this, it's stop, lol. Creative folks aren't we? Hehehe. I agree with sourmise that in a Dom/sub relationship where there is a real and true relationship a safe word really never has to be used. Each one should know the others body language well enough. However things happen sometimes and safe words are needed, or at least one needs to be established.
For example, Doc and I both use the word stop, and if we use it during a session then everything at that moment will stop and the session will be over. Again, we have NEVER had to use our safe word, well once he did, but that's a whole different story. The closest he ever got beside this other time was I believe the last time we got together. I got a new paddle, a plain simple rubber grippie ping pong paddle. I had never used this toy before and didn't know how it would feel. So I smacked him once and he gave me this look. Then I smacked him again and he said in a pretty deep doctors voice actually, which is what made me know he wasn't kidding, that that paddle really hurt and not in a good way. While most Dom's would not have put up with that kind of back talk, I do when it comes to new toys. But it was my lacking in ability and knowledge with that toy and that was not his fault. I continued to use it but he did after a few more swats start huffing and puffing. He really doesn't want me to use that toy again, but honestly, you sure bet his sweet cute little ass I am going to use it again, as a matter of fact I can't wait. Besides that, he's got a few good ones coming to him anyway which are not meant to be fun, he will pay and that is just the paddle I'm going to use whether he wants me to, or likes me to or not. He has one coming for something that he did that was so very wrong in fact that I will probably gag his mouth, put his ass over at least one, and maybe both of my wedges then tie him to my 4 pount restraints on the bed so he can't use his safe word and he cant wiggle out of getting the punishment he so justly deserves.
It's these kinds of spanking I don't really like to give. It's like spanking one of your kids, and it will probably hurt me more than it will hurt him, at least mentally speaking. Actually it will probably be me who's crying by the end of his spanking rather than him. Then we will need some good recovery time for both of us. But the great things about giving this kind of spanking is that once the spanking is over it's over and so is the crime. I can start healing from this and finally be able to put it away. But spaking him hard over the things that caused me pain or hurt is something that he expects and probably wants. Doing things this way allows instant forgiveness for both parties involved, so it can finally be dealt with and settled, never to be brought up again. Although to be honest, after his spanking on this matter, I am quite sure he will never do it again. And in the off chance he does it again, he will get it again only harder, but the first one will have already been forgiven.