The 1st guys under estimated the weight of the chuck they removed. By a lot.
The 2nd guys just made a error in judgement. A foreman decided that they could drive across my neighbours field to set the next upper section, the operator said nope, I need plates to drive on. Foreman says nope, I'll show you how to do it. Couple hundred feet later he hit a soft spot and the crane flipped over backwards, instant $2 million check wrote. The crane was new in July and is now being cut into little pieces. I have some up close pictures but can't post them. No one was hurt and they have 2 more cranes to practice with.