My experience went the other way.

I bought a Y-class walk-up ticket at YYZ for YYZ-YOW, but my YOW meeting was cancelled, and I only had 30 minutes to catch the next flight to MIA, so I flew back from YYZ-MIA and went straight to the ticket counter at MIA to have them process the refund.

The refund was accidentally put through as $U.S. last year when the $CDN was trading at about 63 cents U.S., so my credit card was credited for about $250 CDN more than I paid for the ticket.

To date, no one at AC has caught the mistake.