Recently, Master "punished" me for forgetting to put my collar back on after changing my clothes. He bent me over and spanked me with a ruler, which is nothing new; I enjoy being spanked. Then, before he started to fuck me, he told me that I could not orgasm, no matter how much I wanted to. This was new, and a challenge for me because I orgasm very easily. It wasn't until the very end that Master gave me permission to come, and when I did it was one of the most intense that I've had.
This was the first time that Master had ever "punished" me, and I thought he did an excellent job. It wasn't a real punishment, since he wasn't really mad and I actually enjoyed the experience. Nevertheless, even though I found it pleasurable, it was still a challenge. It took all my self-control to keep from coming before he let me, and that challenge (as well as the uniqueness of the experience, since we'd never tried orgasm control before) reminded me that I should remember to put my collar on.
The spanking was "punishment" as well, even though I enjoy it and we use it on a semi-regular basis, because it's the kind of thing that is usually used as punishment instead of a reward outside of a BDSM context. You never spank your kids to reward them for good grades. This brought home the meaning of "play" for me. We were both pretending he was punishing me, and to an outward observer it might have looked like that, but we knew the truth: we were just playing around and enjoying ourselves.