I was told the following story when I visited the local (3000 MW) nuclear plant in mid Feb.

Previous week the plant had been running at maximum power, since it was way below freezing and many houses in Sweden are electric heated. Someone had scheduled repair of a non-critical fault to the outgoing 400 kV lines during peak load hours. (Great idea!) The contractor sparkies went out to the disconnects outside the plant and started to shut down the lines - WITHOUT INFORMING THE CONTROL ROOM!!! [Linked Image]

Next thing, the reactors run wild since they have nowhere to put the power. Automatic emergency shutdown. The first reactor shut down before the guys in the control room had figured out what was going on and managed to stop the sparkies. Luckily, there was enough headroom on the national grid to compensate the loss of one reactor. The loss of two reactors would have meant a nation wide blackout...

(I hope the subcontractor had a good insurance!)