My AA story:
Once I had to make a last minute business trip and because of time constraints the only flight I could take was an AA flight. Needless to say, the ticket was a very expensive ticket. I had an upgrade voucher (from Sheraton Club Intl.) for AA which I tried to use. There was space in the first class cabin, but the agent said that my ticket was ineligible because my travel agency got a 10% discount (of a super expensive ticket) from AA. Fortunately I read the fine print on the back of the certificate and showed the agent that the booking class of my ticket was listed as eligible, so she reluctantly upgraded me.