[KevinH]

Either way you end up in the LoadFile() routine in MainWindow.cpp and the exception (a runtime exception) is caught here:

  } catch (const std::runtime_error &e) {
       ShowMessageOnStatusBar();
       // ImportHTML/ImportEPUB use wait cursor and can throw exceptions caught here
       QApplication::restoreOverrideCursor();
       Utility::DisplayExceptionErrorDialog(tr("Cannot load file %1: %2")
                                             .arg(QDir::toNativeSeparators(fullfilepath))
                                             .arg(e.what()));
   } catch (QString& err) {
       ShowMessageOnStatusBar();
       // ImportHTML/ImportEPUB use wait cursor and can throw exceptions caught here
       QApplication::restoreOverrideCursor();
       Utility::DisplayStdErrorDialog(err);
   }

    // If we got to here some sort of error occurred while loading the file
    // and potentially has left the GUI in a nasty state (like on initial startup)
    // Fallback to displaying a new book instead so GUI integrity is maintained.
    CreateNewBook();
    return false;
}

And a new empty epub is created as a result in an attempt to keep going.

So for some reason, this code is not happening when launching sigil by double-clicking on a epub on Windows?