Domain front running is when "insider information" is gathered by a party from monitoring attempts by an Internet user to check the availability of a domain name, and then that info is used to by that party to register that domain name.
Several video-oriented websites received illicit boosts in their traffic from third-party ad popups that occurred on people's PCs. Those machines had been infected by spyware, which powered those popups.
Spyware researcher Ben Edelman looked into pay-per-click scams and the lengthy AdWords Content Policy, and found plenty of advertisers pushing to and past the boundaries of "ethical and legal advertising" online.
Querying for risky keywords puts the searcher at risk of being sent to a site that could prove malicious, but even queries for popular keywords poses some risk too.