A company I worked for had a leave booking system built in Lotus Notes (iirc). If you booked Friday to Monday off, it would take four days off your available leave.
Reporting the bug didn't make any difference, of course.
We soon discovered that putting Monday as the start date and the previous Friday as the end date gave us four extra days of available leave.
Shortly after that the system was scrapped and we upgraded to drawing on a wall-planner.