Errors can also occur due to omissions in the specifications (the document that lays out what a new system is to do).  Although a lot of time and planning usually goes into the design of any new system, there may very well be certain situations which were never considered at the design stage.  Designers must always assume that if an error can be made, an operator will manage to do it, whether on purpose or by chance. 

One computer system tried to raise the temperature in a chemical process to 800 degrees.  The operator had only meant to enter 80 but accidentally added an extra zero with catastrophic consequences -  a poisonous chemical was released into the atmosphere. 

Another example of such an error occurred when a patient received a lethal dose of radiation treatment for a cancerous tumour.  The computer-controlled equipment had previously been used hundreds of times without causing an error.  On this particular occasion however, the operator accidentally altered the data he had entered by using the cursor keys in a particular way that had not been anticipated by the system designers.  The patient died.