For brands competing to promote similar products, the rivalry can often lead to aggressive creatives. But when you add user-generated content into the mix, the results can be volatile. Witness a recent lawsuit filed against Quiznos in which Subway claims that UGC created on behalf of the defendant ran afoul of U.S. law.

The case, which is set for trial sometime next year, centers on a 2006 contest run by Quiznos on the iFilm website. In the contest, Quiznos asked its fans to depict its brand of sandwiches as being superior to those made by Subway.

In the suit, Subway alleges that the videos depicted its brand in a "disparaging manner" and that the films contained "literally false statements."

While there is always an inherent risk that users may create objectionable content for brands, this case raises a more troubling issue for marketers, according to Brad Brinegar, CEO of the McKinney ad agency.

"Let's just hope that as collateral damage it doesn't kill the entire genre of competitive advertising," McKinney told The New York Times.