I was friends with someone during sixth form - it was a toxic, one sided 'friendship' on reflection, but I'd always struggled with friends, so I was ready to believe any old bullshit.
Most of the lies were pretty innocuous really (the using and underhanded-ness was worse), except for one - not long after I'd met her, she told me she had multiple sclerosis, diagnosed a couple of years earlier when she was 14. Which is plausible, except she didn't have it at all - she made up emergency hospital visits and everything (usually when we had plans, as 'reasons' for why she hasn't turned up). Then, she just stopped mentioning it after about 9 months of me knowing her - never mentioned a medical appointment or any difficulties again.
This was made all the worse by the fact that a girl at our sixth form was diagnosed with MS when we were 18, so it was even more apparent she didn't have it.
A lot of her lies were proven to be so once I'd met her family - who were lovely! - and I don't think I'll ever really know how much of what she said was the truth or fiction.
The worst of it was, I never called her on any lies - even when I knew that's what they were - because I didn't want to lose a 'friendship'. I think I was just an easy touch in retrospect.