Zbrush retopology bug

Artist_in_a_box

Does anyone know of a bug in Zbrush that causes your retopology to be deleted? This has happened three times to me now while retopologizing, I cannot seem to find a cause, just that randomly some of my work will vanish and the whole application will revert to how it was a few minutes ago. What I mean is that as I am working I will retopo more of the mesh but then this bug will occur and my work will vanish and I will be back to where I was half an hour ago. Even ctrl+Z will start undoing work that I did half an hour ago and not what I just worked on. It is very odd and annoying. Anyone else have this kind of bug?

Cheers