Comment 2 for bug 211735

> Start a new Inkscape document, add something to it and save it.
>
> From a separate command line window, chmod the file so that it
> is not writable: "chmod -w foo.svg" should do.
>
> In Inkscape add some text that says "spong", thereby changing the
> document.
>
> Save it. Note that the save fails because the file cannot be
> written.

This has been fixed: see bug #353847 “Save over a read only file doesn't turn into Save As...”