It can sometimes be hard to count the number of failures for which you are prepared, and sometimes the preparation for one overlaps with the preparation for others. Additionally, just following the safety rules of basic cave diving involves planning for multiple failures.
For example, when I was trained for cave scootering, I was hammered with the rule that I should always assume that the scooter will fail at the worst possible moment and therefore never take it anywhere I can't swim out. That brings gas planning into it--if it does fail at the worst possible moment, then I will be a swimmer at that point. I have to plan enough gas for that swim out, and I don''t mean exiting on my last molecule. Thus planning for one failure requires planning for another.
But what if my scooterless exit suddenly includes a lost light? No problem--I always have at least three lights. That's a standard procedure. So I am really planning for three failures, aren't I? In fact, the safety reel on my butt D-ring, the extra spools in my pockets, etc. are all routine plans for failures on every dive.
But I also agree that at some point you have to realize that you can't plan for everything and play the odds. We accept a certain level of risk every time we get out of bed, and if we stay in bed long enough, we are risking bed sores. In a recent thread about flying after diving, someone brought up the fact that on average, 40-50 airplanes lose cabin pressure every year. He was indicating that this was to him an unacceptable risk for flying after diving. I looked up the total number of flights each year and calculated that the likelihood that the plane you are on will have a depressurization incident was 0.00000114%. Call me crazy, but it is a risk I am willing to take. Other problems--loss of scooter power, light failure, siltouts, etc.--happen with enough frequency that I will prepare for them every time.
