I opened this thread thinking of things people do and say that makes it obvious they're insecure, and this, this is exactly it.
I have a friend who in our teen years would make up grand stories about so many things, I guess because she thought she wasn't interesting enough for us.
First she made up a story about a "boyfriend" she had from swimming class, but she made the mistake of telling us that he was related to some of her family friends. At a family party where she had brought her best friend, the friend asked about him, and it turned out to be one big lie, which we had suspected for quite some time. The family knew nothing about "the guy".
Maybe about 6 months later, she suddenly became part of a friend group about 3 years older than us. We knew it was bullshit almost from day 1, but we didn't think it would become as extensive as it ended up, which is why we never called her out. She would receive "phone calls" and delete their number straight afterwards because she "didn't want us to get ahold of the number and embarrass her". She told us about how she sneaked out the window at night to go to parties. One of them became pregnant, then got an abortion. One of them gave her her first kiss. When we finally decided it was too much, she was actually starting to ease out of the lie, so we never did end up confronting her the second time.
It's been several years now, but when I see her every once in a while, I still can't help but be sceptical about the stuff she says. I suspect that she still makes up smaller stories of things that has happened to her, but fortunately nothing of the scale she used to.