Here's my recent "war story":
6/24: Buy tickets on Delta Express nonstop from Columbus, OH for $202.50 apiece, 10:55AM departure on 12/7. Due to cancellation experience last year I check the itinerary online several times a week.
8/5: Return from weekend trip to discover Delta has dropped Delta Express service from Columbus and replaced it with flights on Chautauqua Airlines (never heard of them) using Embraer commuter planes (hate them). We are rebooked on a 7:00AM departure. (We live near Dayton and have a 75 minute drive to Columbus Airport.) I call Delta and verify we can get a refund, because our time changed by more than 90 minutes (actually the least of our problems). I begin investigating alternative flights.
8/6: The very next morning I discover that Delta has raised prices on alternative flights by $30! I call to get a refund on our Delta Express tickets.
8/6: Later that same day..., I discover that Southwest is offering a short-term promotional fare of $146 nonstop from Columbus. Flight times are lousy. Checking Travelocity, I see that America West is matching Southwest fare on a nonstop with better flight times. But the America West website does not show the reduced fare! Call America West reservation phone line. It is so backed up it doesn't even put me on hold! "Try again later" it says. Call web site assistance line to ask why web site doesn't have good fares. Person who answer offers to book fare! I take it and run, saving $226 on 4 tickets!
Eternal Vigilence is the only way.