A side-effect also has a meaning in programming.

A procedure has side-effects if it modifies variables other than its inputs and/or outputs. This is bad 'cause it makes you code not entirely clear to the reader (unless it is heavily commented... ya, right. That happens...).

A plain old side-effect isn't too bad. You can usually hunt them down by looking at public/global variables. However, sometimes side-effects are used deliberately as a way of having two procedures talk to each other. This is really bad, and almost impossible to debug. I call it communication by side-effects, and I hate it.