Human nature encompasses all the physical, biological, social, mental, and emotional characteristics that define human tendencies, abilities, and limitations. One of the innate characteristics of human nature is imprecision. Unlike a precise machine—each time, every time—people are imprecise, especially in certain situations. For instance, humans tend to perform poorly under high stress and time pressure. Because of “fallibility,” human beings are vulnerable to external conditions that cause them to exceed the limitations of human nature. Vulnerability to such conditions makes people susceptible to error. Susceptibility to error is augmented when people work within complex systems (hardware or administrative) that have concealed weaknesses—latent conditions that either provoke error or weaken controls against the consequences of error.