I was driving from South Carolina to Virginia, I was completely broke, and had exactly $20 in cash, and only a couple dollars in my checking account. I did my math wrong, and didn't have enough money for enough gas to make it home. I was trying to draft behind semi trucks and drive slow to conserve fuel, but it wasn't enough.
I called my bank at the gas station with my needle on empty and asked what would happen if I overdrew my account by $50, and the guy on the phone asked me to explain the situation. Afterwards he said I was good to go.
I asked, what does that mean? He said there's now $50 in your account. You can use it to fill up your car on your debit card.
I filled up my car and made it home. When I checked my account later, expecting to see an overdraft fee, there was a deposit of $50 from some account I didn't recognize. The guy had just transferred me $50 from his own account. I never figured out who this was, so ~18 years later, I'll take this opportunity to say: thank you sir.
I don't know that this is THE nicest thing anyone has ever done, but it was a small thing that made a huge difference in that moment.