Usually when you take any kind of math oriented exam (math, physics, mechanics, etc.) You get a pack of problems slapped on your desk and you finish it in the time allotted. So you can generally blow through 90% of a test and get stuck on one difficult part for the remainder or just have that "A-ha!" moment at the end to squeeze in a few extra points. But this one guy had the bright idea if handing out and collecting each problem at 15 min intervals. So once the problem was collected, that was it. Even if you figured out the mistake earlier, it was already too late. Hate the prof to this day. He wrote about it on scamfighter.net