Buying a Parking Ticket

I went to buy a ticket for the car park today. I inserted my pound coin and waited for ny 40p change and parking ticket. However instead I got 30p, a parking ticket and a credit note for 10p. It seems the machine was out of change. Is a parking meter allowed todo this, I didn't get prompted if I found it acceptable to get a credit note. My credit note just metions the value, nowhere does it say where to get your money back. Bah enough moaning.