What was the nicest thing a stranger has done for you?

About a year ago, I had to get gas after work. Since I was short on money at the time, I could only get a small amount. Anyway, as I told the gas station attendant how much, a guy behind me spoke up and said he would put five dollars more towards what I had. I thanked him graciously and went to pump my gas.
What was the nicest thing a stranger has done for you?
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