I heard about an error, which I have not been able to reproduce yet. I can't reproduce, because I don't have a source in which the error is. I have asked for the file though.
It seems that Sigil will crash when a word contains the HTML entity . That entity is not visible, but apparently the word is indicated is misspelled. When after right-clicking on the word the correct spelling is chosen, Sigil crashes.
I will try to reproduce, but at the moment if I use it doesn't get converted to a HTML entity.