How often do you test your redundancies?

Hey all. So, we all have redundancies on our network (if we don't, we really should get them). Whether it's a redundant router, redundant sup module, redundant cables in a LAG, or redundant ISPs, we have plenty of redundancies.

But, just like backups, if you don't test those redundancies, you might as well not have them, because you don't know if you can count on it.

Testing could be as simple as unplugging one strand of fiber to simulate a unidirectional link failure, or yanking the power cables on your core router to simulate a power failure. Or, you could simulate a failure scenario of a specific feature, like what happens if an access switch sends a superior BPDU?

We can all say with reasonable certainty what SHOULD happen in a specific scenario, but how often do you test to see what DOES happen? What testing methodology do you use?