We take it for granted that we can stroll up to an ATM and take out cash whenever we need it, but when storms knocked out power in the Bethesda, Md. region this summer, that simple transaction became impossible. Of course, businesses that normally accept debit and credit cards couldn’t do so for the same reason, which meant a lot of people were stuck — an uncommon but unnerving side effect of our increasingly cashless society. A mobile Bank of America ATM rode to the rescue, parking near one knocked offline by the power outage.