Back when Microsoft was literally paying people to use Bing, bing had a promotion such that certain items were eligible for a 20% discount, which was paid via a rebate check from Microsoft.
One day, I ordered some item then canceled it, and a couple months later I got an email informing me I had been credited for the purchase via MS, and my rebate account had, $10 or something. It took me about... 30 seconds to realize the implications of this.
I ordered as many plasma TVs as I could, one at a time, and then canceled them. I got about $900 of rebate checks before someone got suspicious. They canceled my account because I was buying too many TVs. They never did figure out what was going on.
Was it ethical and moral? No. But I hate MS and they've stolen thousands from me over the years. It felt great to get one over on them.
Edit: I've hated MS since I saved all summer to buy MS-DOS 5 and it was complete crap.