That's a good point about testing. I think the key point is where the testing is happening. I'm sure the UPS (and other power equipment) is tested by the manuf

That's a good point about testing. I think the key point is where the testing is happening. I'm sure the UPS (and other power equipment) is tested by the manufacturer for many different failure scenarios. The issue is that to the end user, the response is often surprising. So I'm sure to the APC engineer, having the unit shut down when the battery fails (according to some metric) but while there is still mains power available is something that they tested. But the consumer hasn't tested a battery failure, and in fact it may be difficult to test the exact failure mode (low voltage etc) without access to some spare, old, batteries. So when it happens and you're surprised and you call the manufacturer, they (almost always) will say that it operated according to how it was designed and tested. It just didn't do what you wanted or expected. At least that was always my experience doing a post mortem on a datacentre outage.