Good to remember, though I agree with the above, it should be 'Plan FOR failure.' The planning to fail is a funny little catch phrase but enough of those in the work place can get you down (or turn into an enterprise, ie. www.despair.com).
I find it valuable when testing to have my manager make a run. He's really good at messing up the intended use of a program, entering wrong data, attempting unintended processes, and sometimes he'll just bang on the keyboard and hit enter. It's also quite entertaining. But even after all that, there are still failures that we hadn't planned for. Maybe the phrase should just be, 'Life happens, what the heck?'