You're searching and searching for the perfect airfare. Suddenly, you find it. You bookmark the site, check and double check all the plans, wonder if the price will go any lower, and return to the site a few hours later...only to find that fair has disappeared or the flight is now sold out.
Next time, nail down all your plans before you start shopping for airfare. Once you find the fare you like, you can book your flight and move on to other things.