Misspelling a Domain
Back in the glory days of the late 1990's, the web designers had responsibility for the registration of domain names on behalf of clients. One particular designer had a face to face meeting with a major client, during which the client asked him to register Car Tune Central dot com (or so he thought!). The staffer did a check and was delighted to see the domain available. He made the purchase and proudly emailed the client.
An hour later his boss called him in to his office to say that he'd had a call from a very frustrated client who *actually* wanted him to register Cartoon Central dot com. Needless to say the desired domain wasn't available and the whole office dined on his mistake for months.
Letting the Domain Name Expire
Now what type of company would allow their domain to expire a month after site launch? A very large one, that's who, the company’s name is withheld to save the company some embarrassment but the site was offline for a total of 2 days while they scrambled to pay their registrar, sort out DNS propagation and cover their tails.
Buying a Dot Bíz When the Dot Com Was Available
Buy the dot com extension of the domain name if it is available. Although sometimes some extensions were said to be the next big things, take them only at the face value. Nowadays, very many new extensions cropped up. But, in my opinion, the dot com extension is the best. For example, some time back, dot bíz sites were rumored to be the next big thing and all companies were being urged to choose them over dot coms. Now also, dot ws, dot info and dot mobi sites are being promoted as the next big thing. But the fact is that dot com is and will remain the most coveted domain name extension for years to come.
Promoting a Domain Name, You Don't Own:
At one time, the University of Newcastle have spent thousands of dollars on television advertising in Australia, marketing their new site for online post-graduate coursework: GradSchool Dot Com. There's only one problem. The domain for this site is actually Gradschool dot com dot au. They don't even own Gradschool dot com!
Sadly, this glaring marketing error seems to have totally escaped them and they are happily referring to their brand as Gradschool dot com on all their marketing material and throughout their dot com dot au domain. It's tragic to think of all the potential students typing in Gradschool dot com expecting to find the University program. I see that whoever purchased Gradschool dot com has slapped up some AdSense code on it so at least somebody will reap the benefits of those thousands of advertising dollars wasted by the University.
Involving in copyright battles
Also, do not use copyrights and trademarks of other companies on your domain name so do your research to avoid getting involved with legal battles. While others may not ask for damages under such conditions the worse that they would do would be to take back your domain name.
Don't commit any of the above mistakes in the matter of domains.