"IP Proofing" ? I've seen this term thrown around, mostly in terms of saying it can't be done, but I don't know what it means..
"Iterative Probability Proofing". The idea is that with enough rolls, you are going to roll three natural ones in a row or otherwise be subject to the whim of bursty probability at some point between first and twentieth levels. And then you die. Dying sucks, so make sure you (a) have the ability to reroll critical rolls and (b) have the ability to avoid making critical rolls in the first place.
Put another way: a 95% chance of getting something right looks like a pretty safe bet. If you make that bet fourteen times, you have only about a 49% chance of succeeding on every roll. With more rolls, the chances of failure approach one.
edit: bah, sorry Bozwevial, I fail at reading comprehension. The answer at the top of this page is perfectly satisfactory.