Tom was discussing his wonderful proposal with his friend Sameer. Everything looked great; details were being worked out. They have done all number crunching over investment and profitability; What a perfect plan and opportunity! You guessed it right that, they were talking over some internet based idea (may be another youtube, orkut or facebook).
Next big thing was to find a perfect domain name for thier operation. They started their favorite browser (i.e. Firefox) and launched a very popular domain registration website (what about X1 Domains).
After hours of brain-storming they found bunch of names but none of them were exact for the idea. All of sudden someone suggested the “perfect name” and it was not taken. But they wanted to discuss more and decided to wait for few days.
After 3 days Tom finally wanted to register the domain but guess what? It’s gone! They now saw a site trying to sell everything from Viagra to “cheap flight tickets” 
Now a web-spam was showing on the very same URL that they wanted to reserve.
Everyone was wondering that this domain was not taken by anyone for years. What happened all of sudden?
YOU LEAKED YOUR OWN IDEA.
It is no longer a trade secret that domain-registrars sell their domain search history to “domain junkies” for a decent subscription fee. These junkies then immediately take any good and catchy names to re-sell it for bigger bucks. Many simply add them to their “pool of domains” engaged in pay-per-click based advertisement.
Moral of story is “after finding your perfect domain, reserve it immediately, never wait“. Also if possible do not use any registrar site to perform domain search queries. If you know Linux, you can even use “whois” command (if not installed, run: yum install jwhois).
A second piece of advice is “consider reserving any similar domain names as well“. As an example, if you are planning to launch next cool hot deals site http://x1deals.com ; you should also reserve http://x1deal.com and related. If you run a non-profit and registering a “.org” then also register “.com” (if available).
It can save you from potential traffic and reputation loss due to typo (traffic generated by typing mistakes) and junkies are hunting for such opportunities.
Hope this helps in saving your next “million dollar idea” 
Cheers,
Sudhaker