That's not a bug, though. While confusing it is how it's intended to be. A tool is essentially your model, but we have to keep in mind that ZBrush is really not just a sculpt app, but a digital 2.5D illustration app. So when you do Ctrl+S it wants you to save the document, i.e. the canvas on which your digital illustration…
alright, if you guys want smooth shaded in zbrush... heh, not a problem for me, but i hope this doesnt make it even bugged software ;) . I hope they also fix a bug when saving a tool. I found a solution for this, instead of pressing ctrl + s, you have to go to the save tool button and save from there (f... annoying).
Hehe, yeah that does sound like a bug :) Does it do that every time without exception when you hit Ctrl+S? I remember mine doing something similar with another button, and I somehow managed to fix it. Just can't remember how, so it might have been one of those things that just "fixed itself automagically"..