I worked in a project team a few years back that had a lot of vendors sitting in with us.
There was a woman who worked for one of the vendors, who gave me a very bad feeling from day one ... just the sight of her sent a shiver down my spine. She was incredibly sweet - sickly sweet - but I would often catch her giving people the evil eye when she thought they weren't looking. A couple of times when we were both working late, she'd let rip and tell me how she hated her job and would start listing what was wrong with each and every one in the project team. It was so bizarre - very Jekyll and Hyde-ish.
We sat with our backs to each other and a couple of times I would feel her watching me, which would be proven to be the case when I spun around. I'd ask politely if she needed me for anything, and she'd just grunt.
When I took leave, she made an appointment with my line manager (who didn't sit with us, nor did he have any sort of involvement with her as a vendor) to express an interest in applying for my job if I ever decided to leave! She also made out to him that I was extremely unhappy. This was all lies - I kept my conversations with her to a minimum and never expressed dissatisfaction with my job. I only found out she'd said all this when he invited me out for coffee and said he had made some initial progress with getting me a transfer to another project. I was absolutely furious and told him that I thought she was untrustworthy and out to cause trouble. His response was to ask if I was jealous of her because she was younger; I honestly could not believe he asked that, especially as I'm someone who is very appreciative and respectful of rising talent.
I moved on - to a new organisation - a few months later, joining a project manager I'd previously worked for at this place (who couldn't stand our line manager). At that point, the vendor woman did take up my old role but was let go after a few weeks for telling porkies in an effort to cover her butt on something quite major! She'd also been acting like a complete witch to a couple of the project assistants, dumping work on them at the 11th hour and blaming them for her mistakes. #Karma. My former line manager asked if I would come back and admitted he should've listened to me, but I had great pleasure in telling him to take a hike (nicely, of course).