A recently found Facebook bug lets you to post un-deletable messages to friends’ walls.

Here’s how it works: Once you’ve blocked a person on Facebook, he or she can no longer see content you’ve posted on the social network, including posts you’ve made to their own wall. Since the content is no longer visible, it can’t be deleted. The only way either party can see the post again is if the block is removed.

In a way, Facebook lets you know that’s going to happen when you block someone in the first place: “Blocking means you won’t be able to see or contact each other on Facebook,” reads a message that pops up when you decide to block someone (see screenshot below). However, it’s a one-sided agreement: The person being blocked isn’t notified, and all of the content you’ve previously posted on their wall remains intact but invisible to both of you.

MORE: Facebook Bug Creates Wall Posts That Can Never Be Deleted.