The thing we clocked in on when I worked at Kmart would round to the closest quarter hour. So by clocking in 8 minutes early, and clocking out 8 minutes after my shift, I got paid for 30 minutes rather than for the 16 minutes. By exploiting this, I was paid 2.5 hours of overtime a week. Cumulatively, during my time there, this added up to about 6.5 weeks of extra pay. I wasn’t ever caught, though.
Edit: actually, since it was all overtime, it was closer to 10 weeks of regular pay.