Ummm... I thought overdraft protection was what got these people into their $30+ fees? If you opt out of overdraft protection you cannot spend what you don't have and the transaction is declined, thus no fees.
Wrong. Happens all the time.
Here is a scenario.
You have $500 in your checking account and make a purchase for $400. The bank is notified instantly and your "available balance" drops to $100 but the transaction hasn't actually occurred yet, bank transactions occur Monday-Friday overnight, they don't happen during the day or on the weekend - this is called processing.
Now, let's say the vendor doesn't actually debit your account for 7 business days. You look at your online banking and see a pending transaction for $400 though but after the 7th day it drops off and your balance returns to $500! Why? The vendor hasn't submitted the transaction yet. So now, like a doofus you think you have $500 because your online banking said you do, so, you go out and make a purchase for $300. This vendor is on the ball and submits the transaction that day, so for today your available balance falls to $200 and the next day your actual balance reflects this after processing, you have an actual balance of $200.
Now the first vendor, finally submits his transaction for $400. Remember, the bank ALREADY authorized this transaction over a week ago because they saw you had enough money in the account. So processing happens, the bank pays the vendor the $400 but wait, you don't have that much! The bank fronts the money for you and charges you a fee. So, now you owe the bank $200 + any fees they tag on you, usually $40 per item.
People run into problems when after they have overdrafted several more items post which are typically $40 a pop. If you don't pay the bank within a week they charged you another $40 for having an extended overdraft. You can easily end up paying a hunk of money for tiny purchases like $5 at McDonald's or $15 at the gas station.
Of course, this could ALL have been avoided if you had JUST KEPT YOUR BALANCE SHEET UP TO DATE!!