[RachelF]

Similarly the disappearing menu items in common software.

Take a simple example: Open a read-only file in MS Word. There is no option to save? Where's it gone? Why can I edit but not save the file?

A much better user experience would be to enable and not hide the Save option. When the user tries to save, tell them "I cannot save this file because of blah" and then tell them what they can do to fix it.

[cosmic_cheese]

The Mac HIG specifies exactly this: don’t hide temporarily unavailable options, disable them. Disabling communicates to the user the relationships between data, state, etc and adds discoverability.

[int_19h]

This has been the norm on every desktop. But lately I don't think app designers know what "HIG" even is. Everything is web (or tries real hard to look like it even when it's native apps...), which is to say, everything is broken.

[urbandw311er]

I half agree. The save option should be disabled, since there is something very frustrating about enabling a control that cannot be used. However, there could be a label (or a warning button that displays such a label) explaining why the option is disabled.