This was kind of fun as there was a site that released a bunch of coupons where it would appear that you could stack them. As a result, the checkout displays a pretty hefty discount as you can see here:
Now I am pretty sure this is an oversight as stacking all these coupons result in like a 50% savings on virtually everything that you dump into the cart. It does make me think of extreme couponing though where people do it at stores all the time. The interesting thing is a lot of people don’t even think to do this even if there are no guidelines saying one per family or transaction as an example. Doesn’t hurt to try.