I recently made a change to one of the credit card auto pay, which brought me lots of troubles.

Not only did I miss a payment, but my 12-month 0% introductory offer also got cancelled, and I was hit with the interest charges. 

When I made the change to the auto pay, I merely changed the payment date.  However, I neglected to double check whether the change would be applied to the current bill or not.  That mistake cost me a phone call the bank and explained to the account manager what had happened.

They removed the interest charges, and sent in a request to restore the 12-month 0% introductory offer.  They did warn me that I might see another interest charges, and I would need to call them back to take care of it.

The auto pay feature is nice and convenient.  However, always double check whether the payment is applied to the current billing cycle immediately before logging off.