You're right that "I didn't read the contract" is an invalid defense in the general case. However, this is an unusual situation. The peculiar circumstances would likely lead the bank to believe they were signing a different contract which they had in fact read. And there is (arguably) some willful deception on the customer's part.

In other words, it's not just carelessness on the bank's part. That alone would not be a sufficient argument. Rather, it's carelessness coupled with the customer's willful exploitation of that carelessness. That may be enough to tip the scales in the bank's favor.