Well, I found a new thing that will need update in the ongoing revision of my book: Files handling by GIMP.
In my book, I indicated that the GIMP-proprietary format XCF (from eXperimental Computer Facility) was not really a great success until version 2.6. Upon launching the command "Save...", you could select any file format to store your images, just by writing the desired extension.
GIMP would then show the related dialog window, to fix the required parameters (compresion, etc.) as desired.
In version 2.8, it is not like this anymore. "Save" command will only write files in XCF format, whereas all other formats will be produced by means of the command "Export as...", that can also be found in the File menu.
While XCF format stores virtually all information in an open image (layers, selections, channels, transparency, paths, guides...), it can be a real pain, if you are used to the "Ctrl + S" shortcut to store ongoing changes... On the other side, it may be a good idea to keep an "original" XCF file with all data inside, and then only export JPG versions... what do you think?