This just happened to me this week. I had just made a rather sizeable payment on a purchase, one that I had to call my bank to lift the cap on my debit card for. I didn't realize the cap was per day, not per transaction. The usual cap is $1,500, but I needed it raised to more than 10 times that. So I went to the grocery store afterward. I got my groceries and was checking out and my card was declined, saying I'd reached my limit for the day. I was embarrassed and told them why it was declined, which I'm sure they didn't believe. I didn't have enough cash on me to pay the $75 and change, so I was ready to leave my groceries behind and walk out the door when the little old lady in line behind me spoke up and said she'd pay for me. She didn't blink when they told her it was $75. I insisted she not do that, but she was more stubborn. I asked for her address so I could mail the money to her. But she said, "No, just pay it forward." I'd never had that happen before.
Another time, I was in line at the grocery store in a similar situation. The young woman ahead of me couldn't pay because her card was declined. She excused herself to go outside to get a different card, presumably, and I told the girl at the checkout counter that I would pay it. It was $56 or something like that. I got a receipt and had second thoughts because the lady had not come back yet by the time my groceries were scanned and the bill presented, so I asked the clerk about possibly getting my money back if she didn't show back up. I wanted to help the lady, not the grocery store. She said, sure, just present your receipt (which she had given me separate from the receipt from my own groceries) at our customer service window and they'll give it back to you.
I waited several days and went back to the store. The lady never reappeared to get her groceries, so I got my money back.
I've given people smaller amounts too, like $5 here and $5 there, mostly people who claimed they had run out of gas. It was probably a scam, but oh well. One old couple accosted me as I came out of a restaurant that I frequent and needed gas. the old guy had a gas can in his hand, so this one seemed legit. I told him to hop in the car, put his can in the trunk and drove to a gas station, where I filled it up. It was like $7 or $8, I forget. It was a small can. I then drove him back to the restaurant. About a week later, I was back at the same restaurant and the owner, with whom I was friends, told me that that old couple had been out in their parking lot all day the previous time I was there looking for someone to buy them gas and/or food. He thought it was a scam.