More on this thoughtWay to lose my sale, buddy. There's nothing more irritating than being made to retype your credit card details because you put spaces in between the number groups. Quite frankly, it's insulting. It's almost like you're calling me an idiot and thus punishing me for such behavior. This is not the emotion you want your users to feel. The last thing I want to do when ordering online is to have to retype my details, even though I know they're right. Another offender during the checkout process is having to enter in the start or issue date for my card. It's just not necessary. In short, don't take a user out of flow if they make a small mistake. You don't even need to notify them of it, if you can fix it in the backend without any hassle.