A bug in an area of the code that was working properly in a previous release, but was broken before the latest one. If we have enough, or one's particularly severe, we may need a point release. Still needs to be marked with the bug label.

Something that we acknowledge is behaviour we don't want, but we can't do anything about it, e.g. because it requires solving the halting problem, comission of a crime, or a target operating system to implement a feature it currently doesn't have

There's nothing to fix - the behaviour is intentional. Tarhiel falls from the sky because Morrowind.esm tells him to do that.