Pedantry

Ah, I got part of that last example wrong. It's dangerous because it will disable the stack
extension, ie it doesn't cause a new page to be mapped at the edge of the stack, but rather
overrides the runtime's automatic mapping of that page, returns to you an result that it isn't
yet mapped, but doesn't restore the stack extension behaviour. So suddenly, and without
explanation, your stack can't grow.

For a POSIX example that's less dangerous but just as unreliable/ useless, try access(2) which
attempts to guess whether you'd be allowed to do something, but can't assure you whether you
will or will not be able to do it if you actually try. It's not even useful for checks like
those in ssh which wants to see if you obeyed the rules for your own safety, since ssh needs
to check access permissions for /other users/.