I worked with a woman for about 3 years who lied about absolutely everything. I made it my mission to prove everything she said was lie

Claimed to have given up smoking… conveniently started taking lunchtime walks just around the corner from work to smoke

I was moving into my first house, she was also funnily enough buying a new house but couldn’t tell me where it was, was being sold privately so wasn’t on any of the property apps.
She had supposedly had double figure miscarriages and was apparently pregnant again and as soon as she was questioned by manager the next day she had a miscarriage but less than 24hours later was back in work with not a word to say about it.
I confided in her a some point that my mum was an alcoholic and then her mum was a recovering alcoholic to the point that if she had another drink she might die. Her mum died during her stint but by that point I didn’t believe it. She took a week off etc. Claimed her mum had started drinking again. Work policy changed not too long after that to state that you may be asked to provide death certificates in order to be paid leave. Wish it had changed earlier as I think it was all BS.
As to the cause… I think it stemmed from being jealous of her sister and maybe she wasn’t given as much attention as a kid as she hated her sister who was supposedly a
witch. She also didn’t have much going for her, no friends to name of, just a partner who was also a bit ‘off’. She was a type one diabetic so maybe she missed out on a lot as a children And then started telling tales to make up for it