Basically, when contingency planning, looking for unlikely, worst case scenarios that technically could happen, but realistically never will. The analogy is you start looking for things beneath the surface of a river in case a log turns out to be a croc and bites you. Trouble is, you are generally looking at your local river that has only logs… chances of there being a live croc ? Pretty slim.
Those tyres are rated for 120 miles per hour, but our car can only do 110 miles per hour tops so they are perfectly acceptable. But what if there's a tail wind, and a hill, and our brakes fail ? says Jack. Good old Jack, always looking for crocodiles (looking for alligators, hunting alligators, searching for crocs etc etc)