Otherwise you'll end up in Internet advertising hell.
If you type in www.powerpuffgrils.com instead of www.powerpuffgirls.com, you'll end up trapped in a sea of advertising windows that includes everything from free music to unsecured credit cards.
And if you want to download the latest version of RealNetworks' audio player, watch those fingers so you don't go to realadio.com instead of realaudio.com. If you do, you'll end up at Casino44, a gambling site.
You'll be the victim of typosquatters, people who have purposely registered misspelled domain names with the intent of making money off your typing errors.
The practice has been around for about two years, when it was pioneered by the pornography industry. Porn hustlers registered the misspelled domain names of popular destinations, like whitehouse.com, and sent careless surfers to their porn sites.
But now the practice is more mainstream, as typosquatters register thousands of pop-culture and corporate domain names.
These cyber entrepreneurs direct the misspelled names to their own sites, where they sell ads to competitors or other companies seeking high-capacity Web traffic.
In 1998 Seattle lawyer Robert Cumbow handled one of the first typosquatting cases when someone registered the name Amazom.com and directed people to a Web site that for a time displayed banner ads for competitor Barnes and Noble.
More recently, the little known OTCstreet.com in August generated so much traffic via misspelled domain names that it had more unique visitors than BusinessWeek.com, according to statistics from PC Data.
But as typosquatters grow their business, they are also raising the ire of companies that claim their customers are being deceived.
In October, a Philadelphia court ordered John Zuccarini, who owns thousands of misspelled domain names, to pay $530,000 in damages to Electronics Boutique. The federal court found Zuccarini, and his company Cupcake Patrol, had violated trademark law when Zuccarini registered five Internet domain names that are "confusingly similar" to the electronics retailer.
Zuccarini, who had said in previous testimony that his typosquatting business earns him up to $1 million a year, did not contest the lawsuit. He owns thousands of other domain names, including powerpuffgrils.com. Zuccarini earns 10 cents to 25 cents each time someone clicks on an ad, according to testimony in the Electronics Boutique case.
Glenn Weiner, the Philadelphia lawyer for Electronics Boutique, said several other online retailers have also sued Zuccarini.
"We've seen more instances of it. As you have big growth in the Internet, you'll have more and more people doing questionable things," said Weiner, who specializes in trademark and copyright-infringement cases and has sued other typosquatters.
When contacted by e-mail, Zuccarini declined to address the allegations against him.
In addition to fighting typosquatters in court, companies can take them before international bodies that regulate domain names. The World Intellectual Property Organization (WIPO) last summer ruled on its first set of typosquatting cases by giving Microsoft, Reuters, General Electric and Yahoo! the rights to dozens of misspelled variations of their domain names.
Most of the time the typosquatters do not fight the lawsuits and complaints. Instead, they register more names or focus on the undisputed ones.
Several sites are known for their use of mistyped domain names, but they change often to stay one step ahead of the companies they anger.
Another company known for registering hundreds of misspelled names is Stonybrook Investments in Belize City. No one at the company responded to interview requests.
In addition to registering misspelled names, like metacrowler.com and cbsportline.com, Stonybrook generates traffic by registering names that begin with www. If a surfer forgets to type the period between www and seattletimes to get to the Seattle Times Web site, he will end up at Casino44, a site owned by Stonybrook.
The same is true if you type in wwwyankees.com or wwwkiplinger.com. Sometimes, however, the destination site changes from Casino44 to other sites, as the typosquatters try to outwit anyone trying to catch them.
Attorney Weiner said he doesn't see the practice ending any time soon.
"Like with trademark violations, they didn't go away because there was a trademark act passed," he said. "There was always someone with a clever idea who would knock off someone else's brand."
Added Seattle lawyer Cumbow: "It's a little tougher than it used to be, but the smart ones can still make money off of it."
Free-lance writer Cynthia Flash covers technology. She can be reached at email@example.com.