What it is: Banks charge their own customers when they withdraw money from other institutions ATMs. Sometimes you get stuck with no cash and there's not a familiar ATM in sight. When that happens, you'll probably be charged twice -- once by the owner of the ATM and then again by your own bank.
How to avoid: Plan ahead, budget and carry an adequate cash reserve. Grocery stores, major drugstores and stores such as Target or Wal-Mart offer the option of cash back when using a debit card to purchase, so if there's one in the area and you need a bottle of water anyway, it could save you a buck or two to get cash from them instead of an ATM.