My favorite ecommerce glitch was a guitar center coupon for $50 off being usable an unlimited number of times. I didn't push my luck and only applied it a few times. Surprisingly they honored it and I got a bunch of miscellaneous guitar accessories for free.
Back in the 1998 there was one website that let you enter negative values into the quantities to zero out the cost of other items. I notified them quickly and they let my order though as a way of saying 'thanks'.
Did this once for credit card rewards redemption along with another glitch allowing one to set the amount of rewards that would be deducted and made out with 20k in gift cards. Lasted a few weeks then died.
When Bing launched, they had a deal that you get 15% off retailer orders if you buy an item after landing on that page from the Bing search results.
Ebay was one of their partners. I Bing'ed "1oz gold coin," clicked into Ebay listings, and bought a bunch of gold for 15% off that I immediately resold.
They quickly added more restrictions to the promotion.
