Quite a few airlines in Europe tack "fuel surcharge" fees onto tickets, especially those booked using frequent-flyer miles. Depending on the airline and destination, the "fuel surcharge" -- which must be paid in cash -- for redeeming miles can approach the cost of just buying a ticket.

For example, if you book a flight from the US to London on American Airlines, redeeming miles, you'll be on the hook for around $10 in taxes. If you book the same flight, using American Airlines miles, but happen to be unfortunate enough to get it on their partner British Airways, you'll get dinged for around five hundred dollars in "fuel surcharge".

Last time I checked, Brazil was the only country where this is illegal.