Electrical contractor says minor interruption
Electrical contractor says minor interruption & doesn't mention to fully disconnect equipment from mains supply - famous last words before the smell of burnt electronics.
Example from my experience but it didn't happen in a datacentre, but it could happen:
While upgrading power from grid to the meter board from 1 phase to 3phase, the licensed contractor approved by the power company (retail/network/supplier) got the wires mixed up so 415v went across circuits designed for 230v - 240v. Anything connected with a ON/standby was damaged. A very costly & time consuming mistake even if some items covered by our/their insurance.