We had a guy working in our branch in DoD who literally did nothing but stare at a wall all day. No assignments, he did nothing. Why? Because everyone was afraid of him going postal. He had a bit of a temper problem.
So, they kept giving him good reviews in hopes that some day he would just go away, but he didn't. Over 10 years pass by and they finally decided that he needed to go, but the paper trail said otherwise. So, they had to document every instance of his inappropriate behavior. When he finally told his boss that he could kill him, they had enough evidence.
My only interaction with him, was NOT saying hi to him one day... to which he repeatedly called me on the phone to say FU and hang up. He then obessed over me in his office with his officemate... He was one wacked dude and getting rid of him was very difficult.
We were also audited and many employees were found to be showing up 10 hours/week, etc... but getting paid 40.