I made the same mistake about a month and a half go.. I checked the ATM and I had about $30 in it..
I ate out and made it $20 even after the tip.
Then got $8 in gas.
Went to Sonic one day, and used a Sonic Card. It was .52 short, so I just popped in my debit..
Well, apparently some charges had not went through yet when I first checked my balance from the ATM. Each charge cost me $30, for a total of $90 in over draft charges..
That .52 at Sonic ended up costing me $30.52!