The glitch appears to stem from a common practice among retailers. Stores won't allow a coupon to reduce the price of an item to less than zero. Free is the best they will do, which is also Target's stated policy.
At first blush, this may sound reasonable. It seems wrong for a store to essentially pay you to purchase a product.
BUT... what happens when the store sends the coupon to the issuer/processor? They get reimbursed the face value of the coupon regardless of whether the consumer got the entire face value as a discount. So this is just a way for stores to make money by scamming their own customers. Granted, this sort of thing doesn't happen often (unless there is a software bug as in Target's case), but it's still a bum deal for consumers.