 My buddy was digging out a pond with a rented excavator, got a little too close to the edge, it was slippery, the excavator slid down and landed on its side and sunk in the mud. He had to break the front window just to be able to climb out and go get help and get to safety before it sunk all the way and was unrecoverable. The hydraulic pressure of the mud and the water mixed together wouldn't allow the excavator to come out on its own.