“If you’re going to panic, panic early!” is a Wall Street proverb that’s cited in print from at least 2000. It means that one should be the first to sell (and get the highest price) rather than the last to sell (and get the lowest price).
   
The proverb doesn’t always work—one could sell and then see the price rise rather than fall.
   
   
Google Books
Meltdown: Asia’s Boom, Bust, and Beyond
By Mark Clifford and Pete Engardio
Published by Prentice Hall
2000
Pg. 253:
Fund manager Gary Greenberg learned one priceless lesson from the Asia crisis: “If you’re going to panic — panic early!”
   
Google Books
Inside the House of Money:
Top Hedge Fund Traders on Profiting in the Global Markets
By Steven Drobny
Hoboken, NJ: John Wiley and Sons
2006
Pg. 132:
That’s the other dictum, by the way, that I’ve learned or that I pass on: If you’re going to panic in markets, panic early. Panicking late is a recipe for disaster.
   
The Motley Fool
How to Panic Well
By John Rosevear
August 29, 2007
A quick search for the word “panic” on one of my favorite financial news sites turns up almost 700 hits, many of them recent. “Avoid the Panic Room,” “Panic in the Crude Pits,” and “Positioning Yourself in the Current Market Panic” are just three of many recent articles that ran on this particular site. Search engines confirm that “panic” is a consistent theme across the major financial media at the moment, with thousands of recent hits.
 
Now, personally I’m inclined to think that Rule No. 1 of long-term investing is “don’t panic, ever.” As I’ve written in the past, following the herd—especially when it panics and stampedes—can lead to big losses during times of market turbulence that aren’t directly related to your investments’ fundamentals.
(...)
Time your panic carefully. There’s an old Wall Street saying: If you’re going to panic, panic early. Like a lot of Wall Street sayings, it’s often nonsense.
   
Coyote Blog
The first rule of bank panics is:
“If you’re going to panic, panic early”
Posted by: Climateer | Sep 12, 2008 4:51:51 PM