Sometimes travel agents like Orbitz, cheaptickets and travelocity may show some non-existent low fares. When you try to book them, they'll tell you that the fare is no longer bookable.

Travelocity may even give you the false information that the ticket has been successfully booked. Once on travelocity I found a super cheap fare on CZ to China, and "successfully" booked it online. Then on the second day, a travelocity agent called me and told me that the airline denied to issue the ticket because the fare has been gone.