I get to edit my original response. We went to Starbucks this evening and there was a line of people who all paid for each others drinks including ours (4 drinks totally $12). So we paid it forward to the lady behind us :) so yay!
It has never happened to me but I work at a restaurant with a drive thru. There is one guy that always pays for the customer behind them. It usually lasts a couple of cars. Lately though he has been giving us a $50 for his order and tells us to pay for the orders until it is gone.