So customers can place orders at our website, but some of the products are actually handled by a third party. We use a web service to communicate about these orders. Obviously, we need a way to uniquely identify each order, and decided with this other company that we would use a simple incrementing integer.

Last week, something strange happened: we could no longer cancel orders by their ID, because according to the web service, the orders were placed too long ago and were no longer eligible for cancellation. But I knew that could not be true: the orders were from last week. So I checked out database, turns out the ID's are not so unique: some refer to two or three orders. Somewhat worried, I contact the guy responsible at the other company and ask him how that could ever happen?

He: "Yeah, when we restart our server, the counter goes back to 1, you see. I didn't think that would be a problem...".

REALLY?! YOU DIDN'T THINK?