Emily McManus points to “one of the most entertaining academic papers ever written” (pdf):
Our research reveals several possible causes of why users make posts that they later regret: (1) they want to be perceived in favorable ways, (2) they do not think about their reason for posting or the consequences of their posts, (3) they misjudge the culture and norms within their social circles, (4) they are in a “hot” state of high emotion when posting, or under the influence of drugs or alcohol, (5) their postings are seen by an unintended audience, (6) they do not foresee how their posts could be perceived by people within their intended audience, and (7) they misunderstand or misuse the Facebook platform. Some reported incidents had serious repercussions, such as breaking up relationships or job losses.
One of many anecdotes cited:
Sometimes, people accidentally post sexual content. One survey2 respondent said, “I accidentally posted a video of my husband and I having sex . . . I didn’t mean to post it, I had accidentally clicked on the video of my daughter taking her first steps and on that video and they both uploaded together . . . I didn’t know I had posted it until the day after, when I logged on again, and saw all the comments from all of our friends and family, and my husbands coworkers (he’s in the army).”
The Dish 