Expected behaviour
Customers type promotion codes at checkout. Valid codes apply a percentage discount. Expired or unknown codes must be ignored: the order goes through at full price. Marketing campaigns end all the time, and customers keep typing old codes for weeks (SPRING10, BLACKFRIDAY and SUMMER15 are still common).
If coupon orders fail
An order that fails only when a code is present, especially an old code, means the lookup no longer handles unknown codes. Check the latest checkout release; this logic is easy to break when refactoring.