Ah...overdraft protection. Useful if you would otherwise have bounced a check to a third party--but very annoying when the bad check is to your own bank!
My Bank of America checking account had overdraft protection that automatically would cover the overdraft from my Bank of America credit card.
I normally kept that checking account near zero, and would do a transfer from savings at the ATM to cover any checks I mailed that day. One day I didn't get around to doing that transfer and the check I had written that day bounced.
That check was to pay off my credit card bill, which I paid in full every month so as to not incur any interest charges.
Let's assume that I owed $200 on the credit card.
Result:
1. Check is not sufficient. The overdraft protection kicks in, and my credit card is charged $200 to pay off the $200 that was due on my credit card...so basically a no-op except...
2. I'm hit with an overdraft fee on the checking account, and a cash advance fee on the credit card.
The cash advance fee was quite a bit more than the interest would have been if I had simply skipped trying to pay the credit card bill that month.
The bank software really should be smart enough to recognize payments between accounts at that bank, and handle errors such as overdrafts more intelligently.