Software's invisible users

Developers and testers are comfortable with the idea of receiving input fro m human users, checking it for validity, and producing out put for human us ers, However, the human user is only part of: the equation. Other, invisibl e software users also submit input and receive output but Rr e less underst ood and sometimes forgotten altogether. But there is a price to pay for ign oring these invisible users. The first step in proper accounting of these u sers is: to understand their nature and how they can cause software applica tions to fail.