Ideally, you periodically test your ability to failover. But if it doesn't work, well, there's a chance that you just caused a user-facing outage with your test.
I've been involved with a number of failover systems where even when it worked there was the possibility that you might hit a _known_ condition that causes the fail-over to fail.
Pretty scary knowing the product your working on has a couple critical holes in the fail-over that management papered over, which while rare could happen. A lot of these solutions are the equivalent of pull the power on one machine move the disk to the other and power it on. The assumption being that the storage mirror/replication/etc being used to maintain transnational consistency for the "move the disk" part is actually going to be consistent when that happens.
What happens if the failover fails as well?