The likelihood of these occurring in parallel is probably higher than you would expect. Backup systems are difficult to test fully since you can only do it via

The likelihood of these occurring in parallel is probably higher than you would expect. Backup systems are difficult to test fully since you can only do it via a full failover. Without running 100% of your systems via the backup distribution breaker, for example, you cannot verify that it can handle the load. You can only design for it. The full system was probably tested at initial install, but obviously more power was being consumed by live systems than by the original test. Entropy doesn't leave backup systems alone either, so over time they can fail even without use, and it is a surprise to everyone when the "working" backup systems fall over.