What is a bug?

A nonprogrammer friend asked me "What causes bugs?"

My answer at the time was that they take many forms, and one form is that a program is used in a way that was not anticipated when the program was written.

For example, you write instructions for a blindfolded person seated at a desk how to get up from the desk and leave the office. The instructions work great in the office environment they were written for, but try to apply them to a different office, or even the same office after it's been "reconfigured", and your program might literally send someone running off into the weeds or stepping into an open elevator shaft.

Certainly a bug is some kind of disconnect between expected and actual behavior, so you'd have to talk about requirements. But the more interesting question is how the bug gets introduced -- how the programmer can think the job is done when it's not, and put it into a simple "cocktail party explanation" (as my econ professor used to say).

So, what's your cocktail party answer to "what causes bugs?"