Today I stumbled across one the worst UX's for a filter I have ever seen, and yes it was a legacy system.

So there is a screen to show a data grid of all orders in the system with accompanying filters; date, status, free text etc.
And there is also a drop-down that allows users to filter by order number, but the genius that made it figured the best way to allow users to search for an order was to render all possible order numbers inside a single drop-down :| and they are not even ordered!