Racks going silent.
A rack losing its power and then all the UPSes failing makes a hell of a lot of noise before becoming very, very quiet. And then it makes a lot of noise when you make the power come back.
Note to others: Do not, I repeat, do not attach a full rack of servers to one 10 amp circuit, and do not attach the UPS-es to each other so you’ll be sure to overload the whole shebang.
Yeah, and fix stuff, don’t document workarounds when the fix is easy.