Or, when a machine "ate money" and didn't credit, the attendant could credit the machine by putting a red quarter in it - that way they knew not to count that towards the totals when doing collections.
A friend of mine used to work at an arcade, and the rule was never to give the customer the red quarter - you put it in the machine yourself. Also, he recalls occasionally having to paint quarters - laying a roll of quarters out on a piece of cardboard in the back and spray painting them.
-Ian