Thousands of Web users were left agog last month by what appeared to be a print advertisement depicting a young girl, seemingly nude and posing provocatively. A photo of a seductive youth normally doesn't turn too many heads nowadays, especially on the Web, but the text on the image — the BMW used car logo, the Greek BMW Hellas Web address and a sexually connotative tag line that reads, "You know you're not the first" — raised a few eyebrows.

Among the hundreds of comments on the numerous websites that posted the image, you could practically cut the sexual uneasiness with a knife. "Who else feels dirty right now?" asked Digg user meretricis. Users wondered aloud why BMW would associate its brand with, as a few commenters called it, "jail bait." Yes, the ad is coming from a division in the more sexually lenient Europe, they speculated, but surely this is too risqué for a car promotion.

As it turns out, it was.

BMW Hellas did indeed create the image. But the company said it never intended the ad to be seen by the masses. It "was only a creative idea of our advertising agency participating in a local contest among Greek advertising agencies," said Vivi Dalakou, a spokesperson for BMW Greece, in an e-mail. "This visual was never supposed to be used as a BMW campaign and never will be." (BMW wouldn't give us permission to use the image in this post.)

For companies that spend countless hours and billions of dollars every year carefully crafting their public image, rogue ads can be a nuisance. You might recall the Speed Dressing video ad that began flying around the Web this summer. The clip, shot in the style of JCPenney "Every Day Matters" TV campaign, showed a pair of adolescents timing themselves dressing, which some say endorses teen sex.

Like the BMW ad, this video was another competition entry, but JCPenney knew nothing about it. A former employee of JCPenney's ad agency, Saatchi & Saatchi, created the video and entered it into the Cannes International Advertising Festival in June, the retailer said in a statement. "JCPenney does not approve or condone its content, and Saatchi & Saatchi was asked to apologize to our customers and our Associates for misrepresenting our company in this manner," a JCPenney spokesperson said in an e-mail.

Rogue viral ads aren't always troublesome for the company whose brand is exploited. Danish beer distributor Carlsberg Breweries didn't seem to have a problem with Romain Pergeaux's Carlsberg and Mentos video (seen below).

The video, which culminates in an unexpected twist, was shot for Pergeaux's portfolio six months after he lost his job at an ad agency, which wouldn't use the script, he said. "I don't really want to talk about all of this. Carlsberg has been fair and kind to not attack us," Pergeaux said in an e-mail, "and trust me, this would have been a bad thing for us if they had."

Some companies have taken the opportunity to simply buy an ad they like that happened to be created by someone else, as in the case of Apple with English student Nick Haley's iPod Touch video, as reported by the New York Times. But Carlsberg says it has no plans to buy the rights to use the year-old video. "I don't think it is capable to sell Carlsberg," company spokesperson Jens Peter Skaarup said in an e-mail.

Professionals looking to build their portfolios aren't the only ones releasing unofficial ads. Many times it's an eager consumer with a copy of Photoshop or video editing software. There was the Wii Fit girl in May, a T-shirt and pantie-clad, virtual hula-hooping young woman, who unknowingly became a sort of cover girl for Nintendo's new exercise game after her boyfriend posted a video of her on YouTube. Then there was the mysterious poster for "Batman: The Dark Knight Returns," an amateur yet convincing image that tricked a horde of Web users last month. D.C. Comics says it was ultimately a fake.

With fan-made videos and company-sanctioned viral ads becoming almost indistinguishable, it's becoming more and more difficult for companies to maintain full control over how the public sees its brand. But then again, maybe that's not a bad thing. After all, the phenomenon has given us Spongebob dancing to Soulja Boy. Can you think of any better brand association?

-- Mark Milian