Just wanted to chime in and say that I have had the same problem on 3 occasions on the same model, so you aren’t alone. I"ll be working away, when I suddenly notice these corrupted verts. After two weeks of troubleshooting I have managed to finally salvage all of my work. Whew!
I may have been lucky though. I fully UV mapped my low res base mesh before turning it into a ZTool and sub dividing. This allowed me to save off a displacement map, then make a new “clean” base mesh. Since my new mesh used the save UVs, I could subdivide and apply the displacement to get me back in business.
Something to watch: I have come to suspect that these errors may occur when leaving a ZTool in edit mode and switching to another application, or allowing your machine’s screen saver to kick in. It’s no secret that Zbrush doesn’t abide by standard Windows protocols, I wouldn’t be surprised if these corruptions had to do with the way the program handles app switching. As a precaution I have begun to save often, and reinit ZBrush everytime I go to switch apps. (No corruptions since I put this into practice a few days ago.)