When I worked at Starbucks years ago, I saw this happen a lot!
Once at Chik-fil-a, the person in front of me paid for mine, so I paid for the person behind me.
Apparently in Houston, it happened 67 times one night at a Chik-fil-a!
This just gives me another reason to go to Starbucks this afternoon...to be nice to someone!
ETA: We had one lady, when I worked at Starbucks, buy a $10 gift card on a regular basis and tell me to pay for as many people behind her that I can with it!