Murphy can bite you in the ankle any time. One company I worked with seemed to have done most things right, including two mains feeds coming into the building from two separate grids (three would have been better, but there wasn't a third available). What they didn't realise was that in one place in the street the contractors had run the two cables side by side, in a shallow conduit because of some obstruction underneath. Inevitably some fool with a backhoe began to dig a hole in just that place (he should have been on the other side of the street) and chopped both cables at once ... Thankfully the company did have not only a UPS that worked, but also a backup generator that started on demand!