Ok another bug, but it's a kinda bit far fetched behaviour :
- Create new book.
- Add Image (Right click on Images Folder, Add Existing Files)
- Go to code view of Section0001.xhtml
- Put your cursor just after <?xml (first line)
- Menu > Edit > Insert image > insert then your image (Remember your cursor is still just after <?xml
- Try to go back to Book view
- Your xml is not well formed. Sigil won't let you fix automatically or fix manually. You need to terminate Sigil otherwise than using menu cause you can't get out that Messagebox telling you to fix auto or manually.