Sometimes, you can't save the page you want (actually I don't know why this happens). How can I force the Safari to Save As?
Don't know the reason and don't reproduce the problem. But according to this question you should be able to override this by clicking on option at the same time.
If others are trying to save an audio or video file that is opened directly in a tab, try to focus the location bar and press option-return.
If you open an audio or video file in Safari, select Save As, and set the format to Page Source, Safari just saves an empty file.
This can happen when the page hasn't actually finished loading. Try pressing command-[period] to stop it loading, then try saving the page.