I was in a queue at the supermarket checkout buying a few impulse buys to enjoy that night when the lady in front of me had her card declined a couple of times. Embarrassed, she said to leave her shopping and she'd have to buy it another time!
I noticed she had basic items of what looked like a small weekly shop - butter, eggs, bread, pasta, beans, veg. It occurred to me she might not have anything to eat that night so I offered to pay for her shopping and told her I was local and she could pay me back. She was so grateful and the cashier was so shocked/delighted and said he'd never witnessed that happen before.
I gave her my details and true to her word a few days later she posted an envelope through the letterbox with the money back and a thank you card!