See this question on applying probability theory principles in software design.
The question is generally the following: you design some system (say software) and rely on some well-known mathematical concept (say hash function). You know that when this concept is used without caution your system can sometimes fail, however the probability of such failure is extremely low.
You need to evaluate whether you want to alter the design or can just ignore that drawback. Consequences of a failure are usually taken into account when such evaluations are done. For example if a failure leads to a person being mildly offended then it is not that of a problem but if a failure leads to a nuclear power explosion that it is a serious problem.
Now the accepted answer goes like this (all numbers here are exaggerated for better perception): probability of Earth colliding with a space rock is 10E-50 and probability of that drawback causing a problem is 10E-100. You see - Earth colliding with a space rock is a gazillion times more likely. So relax, that design is good enough.
Is that reasoning correct? Can it be accepted at all times?