Is it a bug if a user tries to do something they shouldn't be able to do and the program stops them but doesn't give an error message?

If they shouldn't be able to do it, make sure its obvious, like if a button requires admin or something, gray it out or remove it on normal users profiles, if text is read only make sure it visual looks like that, if the user can't tell what they are trying to do is wrong, then it's a problem.