I had so many in my younger days pre-mumsnet 
Names have been changed to protect the innocent 
Chris in 2003 - body of an Adonis and the face of an angel. Wannabe musician/actor type, lived in a squalid flat writing songs and playing his guitar. I lent him £200 to help him with his 'career' (he did occasional work on Hollyoaks as an extra) then he disappeared off the face of the earth. I was furious and sent him a string of texts demanding the money back and threatening legal action etc. Four months later he replied saying he'd been in prison for benefit fraud 
Strangely enough, he messaged me about five years ago apologising for his crappy behaviour 
Stuart in 2005 - met him in a bar on a night out. Not a looker (think Robert Carlyle) but intelligent and funny. Turned out to have a massive drink problem and a massive lying problem. He lived with his parents and obviously saw me as a meal ticket as I had a nice apartment; he started bringing bags of clothes round and putting them in my wardrobe. I had to bag up all his shite and dump it and him back at his parents' house.
Terry in 2008 - lovely sweet guy but not the sharpest tool in the box. Very generous and kind but oh, he did everything SO SLOWLY. Walking, talking and thinking (unfortunately, sex was the only thing he was quick at). He also had issues with food - didn't eat meat or veg (!) so I knew the relationship was doomed. He actually dumped me (it was a 90 minute phone call during which I fell asleep) as he'd accused me of being somewhere I'd never been. I was so relieved 😌
I have so many more sorry tales, please add yours