Never underestimate the stupidity of users. Or the benefits of a little field validation.
One of the web-based systems I've been developing allows users to enter the customer's fax number and automatically fax confirmation details (using Zetafax). It's been working pretty well, until we discovered a flaw today. One user, tired of obtaining valid fax details from the customer, has just been entering a bunch of nines, but then has actually been attempting to send the fax to this number. The first we learned of this was today, when West Yorkshire police asked us to stop trying to send faxes to the emergency services!