For me it's always been when the engineer didn't know which circuit served who at the DP so he disconnected them all. I've had to go out and shout at them a few times.
For ADSL it used to be hundreds of wires on a small board at my exchange. It was so easy to knock other wires. The engineers used to use long pliers to make sure they only got the ones they needed
And don't just think it happens at the PCP level, I've had a megastream shelf shutdown in error at an exchange on me before which wipes out a load of trunks, I've even had an SDH shelf 'disconnected in error'