Dad and I were putting the undercarriage back on a D9G CAT about 5 years back. The way it was setting we couldn"t get the crane on either service truck to set the track frame on one side so we were using the customers 345B excavator to set it in place. We had it close, but the hard bar seemed to have it in a bind, so I climbed in the hole between the frame and the main part of the machine to give the bottle jack a couple of strokes. I hadn"t given it but a stroke or two when I heard something "pop". Don"t know exactly why, but it just sounded to me like something wasn"t right so I immediately began to move backwards, out of the hole. I was less than 10 feet from the machine when the track frame jumped into the air and started swinging against the side of the machine. It slammed into the machine several times before sitting back down on the ground.
We didn"t find out until several months later that there was a problem with the 345"s pilot system that had caused it to be trying to pick up the frame even though the controls hadn"t been moved. Basically it had rocked the machine a bit forward putting an upward strain on the frame, against the bind. When I hit that pivotal stroke on the jack and the bind left, the track frame was free to jump like it did.
Thankfully I got out of the way as quickly as I did. Had I not moved when I did, at the very least, my head would have been between an 11,000 lb track frame and the other 20,000 plus pounds of the main part of the machine as they slammed together. It scared the immortal crap out of me then and even now, looking back, it"s scary just how close I came to getting crushed that day.