Mark Zuckerberg's sister, Randi, got stymied by Facebook's privacy policy this week when a photo she posted for her friends was shared publicly over Twitter.

Twitter user @cshweitz shared a photo of the Zuckerberg family checking out Poke, the latest mobile app developed by Facebook, early Wednesday.

The picture had originally been posted on Facebook by Randi Zuckerberg, who expressed her displeasure with the photo being shared publicly on Twitter.

"Not sure where you got this photo. I posted it to friends only on FB. You reposting it to Twitter is way uncool," Zuckerberg tweeted to @cshweitz, according to a screenshot of the tweet grabbed by BuzzFeed.

The Twitter user, Callie Schweitzer, apologized, explaining that she follows Randi Zuckerberg on Facebook and saw the picture at the top of her News Feed.

Randi Zuckerberg said she accepted the apology a few minutes later and explained that Schweitzer probably saw the picture because Schweitzer is Facebook friends with Randi Zuckerberg's sister, who was tagged in the photo.

Photos that have been tagged are visible to friends of every user in the photo, not just the friends of the user who posted it. It is one of the loopholes in the social network's privacy feature that some users say should be plugged.

Many of the tweets by both users, including the one with the photo, have since been deleted.

"Digital etiquette: always ask permission before posting a friend's photo publicly. It's not about privacy settings, it's about human decency," Randi Zuckerberg tweeted after the photo was removed.

The incident has turned out to be somewhat of an embarrassment for Facebook.

Users frequently complain about the social network's confusing privacy settings, and with Randi Zuckerberg falling victim to them, some users haven't been shy about voicing their complaints.

"This is absolutely about privacy settings. That person shouldn't have seen it. Your brother's site is doing this to us all!" a user responded to Randi Zuckerberg's tweet.