Oh god where to begin.
I used to go on secondment abroad with work fairly frequently pre-kids, sometimes for months or even years at a time. Was abroad for a few months one year and became friendly with a younger woman who was always sweetness and light when you saw her.
We're out for coffee one day and I casually mention DRelative is coming to our country for a visit. Her eyes light up. "Ooh is there any chance your DR could pick up X product over here and bring it for me? I'll pay him, I just can't get it out here." Not a problem, DR does that, I drop it off, she pays the exact amount, without even a little extra or so much as a drink to say thank you for DR's troubles and she's all smiles and thank yous.
A few weeks later I agree to buy a (different) product for me, her and some other friends in bulk as I lived very close to the shop where you could buy said product. She gives me the equivalent of about £20 for the product, which she thinks will be enough. I get to the shop and it turns out to be closer to £35. No worries, I pay up and drop it off to her. I mention the price difference, she says "Ah I've not got any cash on me but I'll settle up with you next time I see you."
Literally never hear from the woman again. Doesn't answer her phone. I see her out in public once and she breezes past looking straight ahead pretending not to notice me. All for the sake of about £15 
The irony is, all these years later, she came up recommended to me on Facebook and we actually now live in the same town!! I like to think I have a very relaxed "I'll get this, you get it next time" type attitude to money but even I'm tempted to go and knock asking for my money plus interest 