1. I know he’s been acclaimed as the best new playwright in the country, but I think he’s just a flash in the pan No one will know who he is by next year.
2. Short skirts are in fashion again, but I hope they’re just a flash in the pan. I would hate to have to start wearing them again.

The expression comes from an old type of weapon, the flintlock musket. In these muskets, gunpowder was first ignited in a small depression or pan; this powder was supposed to cause the charge in the musket to explode. But sometimes the powder just burned harmlessly, giving off a flash but nothing more.