I think that is too radical, it should display a warning so that the user
can decide what to do. Since Emacs only makes a backup the first time you
save, the warning shouldn't be too annoying.
The warning would happen to every single file you edit with emacs.
And I'm running Windows with an account with elevated privileges (so-called
admin account) with default settings. I guess lots of people are in this situation.
If I run emacs "as administrator", then there is no problem. But this is far too
dangerous in my opinion.
Eli as offered an interesting option : to avoidt trying to set the acl if it hasn't changed.
Which should be the case all the time (or most of the time). Combining the warning