A couple come to mind

The guy who ran a porn server from work. Was finally caught when the internet bill was huge and the cio and a tech tracked it through the network to his port. Waited until he walked away for a minute and when he came back found them on hands and knees about to pull the network cable from the server under his desk. Apparently he was making quite a bit off it too. His first words were 'I'm fired, aren't I?'

A woman who lied constantly. She'd claim we built something other than what we were asked to make, and the only defense was to show her the email where she asked for exactly what she got. We learned to document everything with her.

Funny side story. When I joined, all bright eyed and energetic, they assigned me to support her. No warning. I went on and created some software for her, only to get screamed at an blamed for made up reasons. I thought I was about to get fired.... Turns out my team was kind of being assholes too. They all decided not to tell me anything to see if a fresh unbiased person had issues too.