(This excerpt is from the ancient and venerable Structure and Interpretation of Computer Programs)
In testing primality of very large numbers chosen at random, the chance of stumbling upon a value that fools the Fermat test is less than the chance that cosmic radiation will cause the computer to make an error in carrying out a ``correct'' algorithm. Considering an alogorithm to be inadaquate for the first reason but not the second illustrates the difference between mathematics and engineering.