This Is What Happens When You Cut an Inflated Tire With a Razor Blade [Video]
It should go without saying that it's a bad idea to take a knife to a fully inflated tire.
This guy didn't know it was a bad idea and decided to do it anyway.
What happened next is what you would expect to happen: The tire blew up in his face, most likely causing at least a small injury of some sort. At least he recorded his stupidity so the rest of us can learn from it!
Source: Free Beer & Hot Wings