At one place I worked one of the perks was you could buy their products at ~cost, and the way they'd do this is by sending each employee a load of coupons that

At one place I worked one of the perks was you could buy their products at ~cost, and the way they'd do this is by sending each employee a load of coupons that they could redeem. When I got my coupons through I noticed they looked remarkably sequential: HDGT26YA HDGT26YB HDGT26YC HDGT26YD ... Half an hour later I had a list of 10,000 working coupons with a value of a few million (if you could somehow redeem them all). I emailed some marketing manager who'd initially sent them out and got a snotty reply along the lines of "This isn't a problem p.s. please delete those coupons immediately", then started CCing in people progressively higher up the chain until they decided it was actually a problem. A couple of weeks alter a fresh batch of coupons came out that were better randomised. Not that big in the grand scheme of things but pretty satisfying for an afternoons work.