Originally Posted by Faster
I've also encountered this same problem and it occurs for me when using 'All HTML Files'.
It generally happens towards the end of a prolonged session, eg getting rid of every scrap of Calibre junk.
Maybe it's something related to stack overflow or memory gliches with the 'UNDO changes' facility.
Anyway, my solution is to save the work in progress, close Sigil, then re-open the file with Sigil and get going again from where I left off.
I think you are on to something. Long session...
Many of the quirks seem to happen later on in a session, not at the beginning of a Fresh session.
Like you. I exit and re-launch.