HN has an infamous feature called shadowbanning whereby mods can ban a user in such a way that they can't tell that their account is banned. As far as the user is concerned they are not banned, but the comments they submit are immediately marked as dead when they are posted and hidden.

This feature is supposed to be used for cases such as spam bots or crazy people. However for whatever reason sometimes normal users end up shadowbanned.

You can tell if a user is shadowbanned if you have showdead on and most of their comments are dead for seemingly no reason. Yet they continue to post new comments unaware that they are banned.