Caitlin Seida posted a photo of herself on Facebook dressed in a Lara Croft costume, thinking it was private. It was not. The photo spread, and spread again.
It rapidly amassed some of the meanest comments you can imagine, and sparked debate over the logistics and legalities of Facebook privacy settings.
Because Caitlin hadn't restricted her Facebook privacy settings to "Friends Only," she didn't have a legal right to the photos she posted on the site.
As such, they could technically be used on other websites, and they were. It's unclear how this one went viral, but it was posted, re-posted and re-posted.
Despite her best efforts, she learned that once a photo goes viral online, it's very difficult to scrub it from existence. It's a shame but a cautionary tale.