I knew one at school. It wasn’t just the ordinary ‘boyfriend that lives in another country’ type lies; quite a few classmates came up with that sort of thing. Nope, she had this whole imaginary family, more imaginary friends than real ones, taken to a really weird degree.
She had a group of gay best friends, who lived in Ireland, (a country she had, according to her mother never actually visited). She actually went to the lengths of sending a few of us emails, ostensibly from them, from her email address (‘Oh, we’re so close I just let them use my account’). She also had a huge extended family there, including an aunt who had more children than any woman currently living, including three sets of identical triplets.
She got stung by a bee (on a school trip), then skived a few days off school because she’d had a ‘nearly fatal reaction’ (the sting happened mid trip, in front of several classmates, she was fine both at the time, and when she went home 4 hours later). When she came back, she had her whole arm bandaged, and insisted that this was because it had swollen up to twice its normal size, but the bandages were squashing it back to normal.
I forgot my toothpaste on a school trip to France, and tried to beg a bit off her until I could buy some, her being the only one in my room who had bought a full size tube, not a teeny travel one. She didn’t refuse because ew, she didn’t like the idea of maybe accidentally sharing germs, sorry; no, she refused because her toothpaste was special medicated toothpaste that could kill someone else who didn’t have her obscure medical condition. Yes, it was in an ordinary tube with the logo of a common brand on it, her Mum put it in one so she wouldn’t stand out on the trip. She squeezed it from the special tube with all the warnings on to a normal looking one. Stripes and all.
When a few of us confronted her on one of the more ludicrous lies (her imaginary gay best friend suddenly aged 5 years over the course of a week), she went into full on screaming meltdown, and refused to speak to anyone who’d said anything to her for days.
We actually did keep in touch (she was fine when she wasn’t making weird shit up), and she abruptly dropped the lying a few years later when she actually did something real and interesting.

 
 