Reminds me of a friend who used to run IT for a school, telling me the feature he looked for in educational software was greying out the 'print' button for 30 seconds after it had been clicked. Otherwise, the scenario was: kid clicks 'print', looks at printer, sees nothing coming out (maybe the printer is out of paper, or warming up, or whatever). So kid just keeps clicking 'print'. Result: when printer finally comes online, it wastes reams of paper printing dozens of copies of the document in the queue.