Here is how the bug works. Once you block a person on Facebook they can no longer see content that you have posted to the social network, that include posts you have made to their wall. Since they can’t see the post you put on their wall they can’t delete it. Sure you can’t post again to their wall but they also can’t delete your posts.
Facebook in all fairness does warn users of this possibility. When attempting to block a user the following message displays:
“Blocking means you won’t be able to see or contact each other on Facebook.”
Unfortunately the person being blocked is not notified which means they don’t realize the messages disappeared for that reason in the first place.
Unless another friend mentioned the wall post you have no idea that it exists. The potential to yell at someone then block them means one sided fights and insults that can’t be seen on someones personal space.
Here’s a closer look at the block option via @Mashable: