XP SP3
Started Sigil: Made a few changes in BV on 1 file. Switched to CV. Windows Crash Message popped up.
CV. Did a Single REGEX S&R (changed some text to an entity), Switched to BV to see if I liked the results (Good), Switched back to CV. CHANGES GONE.
Repeated replac and they stuck (and for the rest of the book)
One timers (so far), the bane of Hardware technicians
Reporting in case this has happened to others