I was at a botanic garden. A contractor was doing aeration on the grass lawn areas. They had a walk behind plus a large Z-turn type. It had rained heavily and soil was clay. Then the Z-turn started to do a slope that was steep but regularly mowed. Machine couldn't go up, started to slide sideways, aimed downhill without much control. I saw the operator step off the back, but the machine was clearly too heavy for him to exert any additional control. I thought he was going to jump off. They gave up and left some serious sideways skid marks. Be careful guys. Later I noticed a valve box with a nice round hole in it.