Twitter acquired Vine last year and last week released an app for the iPhone and iPod Touch that lets users capture and share short videos. The free app lets users shoot videos that run up to six seconds long and embed them on their social media feeds. Some are calling the service "Instagram for video."

"Posts on Vine are about abbreviation -- the shortened form of something larger," Dom Hoffman, co-founder and general manager of Vine, wrote in a company blog post. "They're little windows into the people, settings, ideas and objects that make up your life. They're quirky, and we think that's part of what makes them so special."

Twitter Apologizes for Porn

Of course, quirky is one thing. Pornographic is another. The six-second clip in question appeared atop users' home screens. It was covered by a warning notice. The image was removed quickly, but not before making Twitter's "Popular Now" ranking.

This isn't the first time pornography and Vine have been mentioned in the same sentence, though. The video sharing service has faced criticism for the sheer volume of porn users share through its service. Usually, users have to actively search out the videos to find them. This time, Twitter served porn up to the masses.

Twitter was quick to apologize: "A human error resulted in a video with adult content becoming one of the videos in Editor's Picks, and upon realizing this mistake we removed the video immediately," said a Twitter representative. "We apologize to our users for the error."

Better Controls Needed

Greg Sterling, principal analyst at Sterling Market Intelligence, told us all photo and video sharing sites are rife with porn.

"The question is whether there are safeguards to keep it from kids and those who don't want to see it," he said. "This gaffe exposed the flaws in the system and the need for more controls."

It's possible the Vine app could get booted from the iTunes App Store for the incident. Apple previously removed an app called 500px because it was too easy for users to find nude images.

"The app was removed from the App Store for featuring pornographic images and material, a clear violation of our guidelines," Apple said of 500px at the time. We also received customer complaints about possible child pornography. We've asked the developer to put safeguards in place to prevent pornographic images and material in their app."

Apple also previously removed an app called Viddy from the App Store for nudity.

As for controls, Vine does let users mark video clips as inappropriate. Once reported, users have to click past a warning message in order to view the video. Vine has the right to terminate accounts of users whose videos are flagged often.