Many crooks like to take advantage of your unfamiliarity with the currency of the country you're visiting. One common scam is called the "slow count." In busy tourist areas, some cashiers will count out your change very slowly, with confusing pauses, in the hopes you'll just take what they're holding out to you and leave. Usually the amount in their hand is way less than what you should have received.
When you do receive change, inspect it before leaving the premises. In Italy, for example, the old 500-lira coins look a lot like their 2-euro coins. The difference? The 500-lira coins are worthless. Also, try to avoid paying for anything with a large bill. Some cab drivers or cashiers will insist that you mistakenly gave them a one when you've actually handed them much more.
When you arrive at your travel destination, you'll want to change your money over to the local currency. You may notice people on the street or in the airport offering excellent conversion rates, better than the hotel or local bureau. But this isn't the time to bargain hunt. Often these black market exchanges will leave you with a pile of fake money or a bunch of newspaper wedged between two actual pieces of money.
How do you avoid falling victim? Learn what the currency in the city you're visiting looks like. Count your change carefully. Exchange money at authorized centers only. Pay with small bills.