I was flying LAN, and the first leg of the flight, in the US, was operated by American Airlines. When I got to Miami for my connection to Peru, I showed my ticked to an AA employee at the gate and asked which gate the flight would leave. She pointed me to a nearby gate, I went there, waited about an hour, and boarded when they called everyone. They manually checked my ticket and gave me the stub back.
When I reached my seat there was someone already there. He showed me his ticked stub and he had the same seat number as I did. As I was cursing the airline for overbooking, we noticed his ticket was printed by AA, mine was by LAN. Since I had just flown with AA with a LAN ticket, I thought it was OK. The flight attendant returned after a while telling me that LAN and AA both had flights to Peru at that same time and my flight was actually leaving from a completely different terminal.
I had to run all the way across the airport, get a new ticket printed, go through security again to get to the right flight. Since my bags had been checked to the final destination, if the AA flight wasn't full and they hadn't sold the seat I had I might have never noticed the mistake.