[AlanHK]

A related issue: Sigil, on Windows, does not distinguish case in filenames.

Demonstrated by trying to rename a file from x.html to X.html, it will pop an error 'The filename "X.html" is already in use.'
I can work around that, e.g. name x to xx to X.

But I had a few days hairpulling when I made a file that worked perfectly in Windows, had no errors with the built-in F7 check, exported to Kindle OK, but loading to a Mac and iBooks, had missing images.
Eventually I used epubcheck and it told me there were missing references and I realised it meant there was a case mismatch in an img code.

I installed the epubcheck plugin and now use it religiously and won't be caught by this again.

I assume this is due to the underlying OS and that likely Sigil on Linux or OSX will not have this problem. But Sigil really should enforce case sensitivity in filenames so errors like this are not hidden.