Users have reacted angrily after a pornographic video appeared as an "Editor's Pick" on Twitter's new video service, Vine.
The six-second clip appeared at the top of users' home screen - covered by a warning notice.
After word spread on Twitter, the image was soon removed, but had by then made the "Popular now" ranking.
Twitter apologised to users, blaming "human error" for the video's appearance.
"A human error resulted in a video with adult content becoming one of the videos in Editor's Picks, and upon realising this mistake we removed the video immediately," the company said in a statement.
"We apologise to our users for the error."
Vine had been facing criticism over the amount of pornography being shared by users on its service.
But those images were generally only viewable to those who specifically looked for them by using search terms.
By being chosen as an Editor's Pick, the video appeared at the top of screen when users opened the app.
One user, Taylor Winkelmeyer, wrote: "I clicked on the link because I thought the warning was a joke.
"I am furious I had to see something like this. Someone please tell me how to get it off my feed."
The firm later began blocking searches using pornographic terms and deleted some users' accounts.
"We're in the process of changing how users find and view sensitive content," explained a spokeswoman.
"We're experimenting with a number of approaches and will continue to iterate."