Omissions can occur for a variety of reasons, such as forgetting, deviation from a procedure or distraction. Incorrect installation is common, as there is usually only one way something can be disassembled, but many possible ways for reassembly. Reason illustrates this with a simple example of a bolt and several nuts. He asks two questions. First, in how many ways can this be disassembled? Write your answer in the textbox. ... Right, the answer is 1. And now think about the ways to reassemble it, how many ways can you count? ... Well, you better stop counting, because the answer is about 40 thousand, excluding errors of omission!