True. But Data centers usually have an A and B power systems independent of each other for redundancy . Equipment is usually connected to both. The failure of either should not cause an outage. (Unless some one is stupid enough to connect core network equipment to only A or B power)

Sometimes only A power or B power is delivered to the rack (otherwise you could receive different phases).

Power failures also occur sometimes when the issue is on the wrong side of where power is distributed - we've lost power to our servers when maintenance has gone wrong for example.

From my point of view if you end up with mission critical equipment connected only to A or B power then its engineered wrong.

Very true, but sometimes this happens at fault of the facility, not the affected company. I've been in a facility experiencing widespread power issues because customers were incorrectly delivered A+A or B+B power, instead of A+B.

This is why I'm a proponent battery floated of DC Power for mission critical systems.

Exactly. You can only test for these scenarios to a certain extent and even when everything works as expected during tests, things still tend to break during production when shit hits the fan.