Originally Posted by MiamiPrep
After 1-year from the date of the first flight, one could always say that the remaining flights on the ticket have expired. I don't think they can force you to fly.
But they can force you to pay.
Unfortunately the rules specifically say who owes whom what if the contract is violated, and it's not a pretty sight. The wise voices have suggested above that it's unlikely the airlines will target a one-time violator, but timid me would certainly not metaphorically spit in an agent's eye while tearing up my last coupon and littering the shreds across their counter.