So this happened almost 8 years ago when I was working in the photo department of a popular retail store.
I am minding my own business, working on some orders when I see a kid (probably 12-14 years old) zoom past my counter, rolling on his shoes with the wheels embedded in them. I believe they are called Heelys?
Anyways, on his 2nd pass, I tell him he needs to slow down, because I didn't want this hooligan to knock over a shelf or get himself hurt.
This kid totally ignores me, and continues zooming past, and not 10 seconds later, I witness it. One of the greatest moments of instant karma i've seen working retail.
As I am watching him roll away, the front of a shopping cart comes jutting out the end of the aisle directly ahead of him, and I watch in utter bliss as he literally tumbles head first over the cart, landing with a thump on the other side with his feet hanging above his head.
I had to summon all my willpower to not laugh, but when that kid finally stumbled back on to his feet and looked my way, he knew.
The look of pure embarrassment on his face was excellent. He exited the store so fast, the customer who was pushing the karma cart didn't even have time to ask if he was alright.