The dispute is over Google's popular AdSense program, which targets ads to keywords in articles and other content at participating sites. The program complements the traditional AdWords program, which runs targeted ads alongside Google's search results. Ads under both programs generate the bulk of Google's revenues.

The lawsuit accuses Google of defrauding advertisers out of millions of dollars collectively by "redefining the universally understood meaning of an input form left blank."

The plaintiff in the case, David Almeida, had signed up for Google ads to promote his private investigation business in Massachusetts. Because he did not want to buy AdSense ads, Almeida said he left the maximum per-click bid blank, believing "optional" meant he could opt out of the AdSense program by doing so.

Instead, it turned out the AdWords bid applied when he did not exercise that option, and he should have put "zero" into the box to opt out, said his attorney, Brian Kabateck.

"Most of the customers that actually fall victim to this scam are the unsophisticated advertisers," Kabateck said. "The sophisticated advertisers will know better, will know how to do it. These are the little guys that don't have money to lose on a program like this."

Google declined comment, saying it had not yet received the complaint.

The lawsuit was filed in U.S. District Court in San Jose, Calif., by Kabateck, Brown and Kellner, a law firm that has frequently filed consumer-protection lawsuits that seek multimillion dollar judgments or settlements. The lawsuit seeks unspecified damages and class-action status.

Kabateck estimated the unwanted advertising involved brings hundreds of millions of dollars in revenue to Google, though he could not immediately say how much Almeida specifically lost.

Kabateck has tussled with Google before and ultimately joined in a $90 million settlement in 2007-2008 over "click fraud," in which merchants are billed for fruitless traffic generated by someone who repeatedly clicks on an advertiser's Web link with no intention of ever buying anything.

It seems that Google Adsense have already dominated the internet marketing business and is now considered the easiest way to making money online.

The key to success with Adsense is the placing of ads on pages that are receiving high traffic for high demand keywords. The higher the cost-per-click to the advertiser, the more you will receive per click from your site. Obviously, it does not pay to target low cost-per-click keywords and place them on pages that do not receive hits.

With all the people getting online and clicking away everyday, it is no wonder why Google Adsense has become an instant hit.