If you're here in the states (probably the same elsewhere...) and you have a bank account (BofA for me) you'll notice that they are always up to NO GOOD.
I did a little experiment. I had a set amount of funds in the bank, $100 flat. Period! I made 5 purchases totaling $60, charged it to my "debit" and I allowed the transactions to clear! Remember, there's "processing" and "pending". I made sure these were "completed" before my next step. 3 weeks!!! These were done and done!
I then made a $50 purchase in gas, leading to an overdraft ($10 over). What happened next was incredible! The single $50 gas purchase cleared! No overdraft.... but the bank decided to "re-open" the previous transactions I made and overdrafted EACH OF THEM!
$35 x 5 = $175 worth of overdraft fees!
So I called up the bank, told them what happened, and their excuse was "merchants take out money at a later date than at the time of purchase, you didn't have that money to begin with". That's ONLY true if they are still pending or processing... what really happened here was it changed the order in which these transactions took place. The gas only being $50, was paid in full by the previous $60 made by the smaller transactions. The bank then "re-opened" the old transactions as pendings and caused each of them to overdraft instead of the single gas purchase.
The guy on the phone "reluctantly" refunded each and every one, and made it so that the gas was the one that got penalized, as it should have been from the get go.
Solution! Use a reloadable debit card for real world purchases. You should NEVER, for any reason, use a bank debit card... unless it's required (house/apt rent etc). Making small purchases at McDonalds or the dollar store etc is a big NO-NO. A reloadable card will never overdraw.
Remember too, it's not as simple as telling someone to watching their spending. Anything can incur an overdraft. Card sniffers, double charges, online scams (cosmetic free samples), literally thousands of things everyday are out to get you. Banks sometimes offer overdraft protection, but these "disable themselves" after a certain time. No reason! Beware!
There are currently class action lawsuits against the banks in regards to this matter. I've joined one and am promised a very handsome award. But sadly, this will NEVER stop the banks from doing this. Their computer systems are programmed to look for large single transactions that incur overdrafts and replace them with previous smaller transactions, even when they have completed (not pending/processing).
Keep a sharp eye on your money folks and never trust the banks! The banks are simply a "tool" in order to make certain kinds of payments. Never let your money sit in an account. Buy a safe and lock it up (and hide it). You're more likely to lose money at the bank than someone physically robbing your house.