I have had this happen several times - mostly I catch it before leaving check-in and they staple the copon back in.

Happened last in MEL in Aug - was flying MEL-SYDx-AYQ. The check-in agent (coincidentally a friend) had toren off too many coupons. Considering we were chatting I didn't even think to check.

At SYD QP was paged and told that they had discovered that the coupon I checked in on at SYD for SYD-AYQ was actually AYQ-SYD (ie the next one). I said that the coupon must have been taken in MEL. After some time they returned the wrongly torn coupon to my ticket and said they would try to locate the wrongly torn off coupon in MEL.

I think i was lucky this was all on QF. I can understand why CX didn't want a bar of it.

I also don't accept the fact that it's an easy or understandable mistake for them to make - for god's sake there is not all that much to the job of checking people in. Tickets are the currency of the airline. If you can't check the ticket a person is using then you shouldn't really be doing the job!