On our way to Tequila, a semi lost its trailer. It careened across the road in front of us and clobbered another truck on the other side. If we had been 5 seconds further along it would have wiped us ...
We watched a brand new John Deere Tractor doing somersaults off a trailer heading right toward us one time. The driver of the trailer had NO tie downs. He apparently set it on the trailer and assumed the brake would keep it on. :) NOPE, didn't.