OK, I'll share: My safeword, which I've never had to use, is 'ferret'.

I chose it partly as a joke (it's an inside joke for us from long ago) and partly because it's the type of word that's never, ever going to come up when we're involved in a scene.

There have been a couple of times that I might have used it, but my brain doesn't work so well all the time and I couldn't remember it. Go figure.