I have one.
I got together with a colleague after my marriage breakdown. I was 31, he was 37.
We had the usual getting to know each other chats, I asked him if he'd been married, had kids etc. He told me he was divorced many years, no kids.
Six months later, we decide to go away for his birthday, and he sends me half the money. On my lunchbreak, I'm in the office and called travel agent to book it, we were a small office of five women and colleague asks to speak to travel agent after me. No probs.
I get to giving our details, then I have a brain fart, when she asked for full names and DOB, not being good at maths, and with his birthday coming up, I wasn't 100% sure of DP's year of birth, I said I'll call him to check, in the meantime colleague speaks to agent.
Call DP, ask for his DOB, he becomes flustered, asks why I need it. I say it has to match passport on booking, he starts stumbling on his words and I'm losing patience, I say just give me your DOB, my lunch break is over soon.
In a defeated voice, he gives his DOB, which reveals he was about to turn 40! My colleagues hear this exchange and find it hilarious, I'm mortified, I hang up, and on autopilot I speak to agent to finalise the booking.
That night, he comes round. Said he lied as he didn't think I'd go out with someone "so much older". I was really upset, I told him his age didn't matter, it's the lying that upset me.
Over the following years, I find out he lied about where he was born (born in Bedford, said it was London), said he grew up in Borehamwood, where my family live, he didn't. Never had. Said he had a Border Collie when he was young, (I had one at the time) and this turned out to be a lie, he'd never owned a dog.
He used to lie about everything and anything, what he had for breakfast, where he'd parked his car at work, things he'd done before he met me, what his score at golf was that day. Honestly, the most random of shit. But he nearly always got caught out, even without me doing anything to disprove him.
In the end, he lied about something big, I can't recall what it was, but it caused a row. I said I was sick of the lies and he had the chance to come clean if there were any big lies I still wasn't aware of. Provided it wasn't illegal or cheating on me, we could wipe the slate clean and move on from all the lies.
Turns out he had a 15 year old daughter who he never saw. This is after insisting he had no kids. Said ex-wife never allowed him to see her, but that he paid generous maintenance to ensure she was brought up not needing anything. And he kept away out of respect when ex-wife remarried.
I was understandably shocked, we'd been together for 3 years by this point. Naively, I suggested that with the passing of time, maybe his ex had thawed and might let him see his daughter again. So, he wrote her a letter saying how much he missed his daughter and hope she was well.
A couple of weeks later, I get a message request on FB, no idea how she found me, but it was the ex-wife and she was livid.
Turns out my DP had walked out on her and his baby, and without a pot to piss in. Made no effort in seeing his daughter, also paid nothing towards her upbringing and she had to go via CSA. Because he was a high earner, he was (rightly) paying a lot of maintenance. She had remarried, had another daughter, she and her husband brought both girls up. She was angry with this letter from the blue when her daughter was just about to start GCSEs.
She told me he was a compulsive liar, not to believe a single word from hos mouth and suggested I move on to someone I can trust.
When this came out, DP let his mask fall, he was bitter and angry that he paid so much of his salary to a "kid another bloke was bringing up", said he was subsidising ex's lifestyle and how he couldn't wait until daughter finished education so he was financially free.
We did eventually split up that year and to this day I have no idea why he lied so much. In hindsight, I should have dumped him when booking that holiday.
His family were nasty about me after the split, turns out he told lies about why we broke up, making me out to be the guilty party.
What an arse.