On UA about 3 years ago, my DEN-SFO segment was canceled. Fortunately, there was another flight to the Bay Area two gates down, and I got on that flight as a standby.

When the flight didn't post on my mileage statement, I called UA and asked them to credit it. They refused, saying that I wasn't on any of their flights that day. Obviously, the process of my catching that other flight at the last minute didn't work as planned.

So UA gave me two choices: I could either jump through a bunch of hoops (faxing stuff, etc.) to get the miles, or I could have a travel voucher for the value of the canceled segment issued with no further inconvenience to me. Since it was an expensive ticket, it was an easy choice. I didn't complain, but I sure thought it was odd considering I was just asking for a couple thousand FF miles.