If you try to flag a post that has been deleted since you loaded the question page, the system returns a rather unfortunate error message:

You don't have permission to use this type of flag

Actually, the problem isn't that you don't have permission to use the type of flag (in the particular case that happened to me, a spam flag on a question), but rather that the post has been deleted.

If the post has been deleted in the interim, the message should reflect that fact. So for example:

This post has been deleted.

Or, to be even more explicit:

You cannot flag this post because it has been deleted.