When we had the building UPS replaced several years ago, the contractors accidentally left one of the neutral bars disconnected... everything was up and working

When we had the building UPS replaced several years ago, the contractors accidentally left one of the neutral bars disconnected... everything was up and working fine, the switch was flipped to cut off mains and ensure the backup system could take over, and suddenly there were pops and smoke all over the place as hundreds of desktops, servers, network gear, etc got a dose of ~430v instead of 240. That was a very expensive test.