Minecraft wiki entry for Block_update, associated text: When a block is placed, it maybe checks and adapts itself to its surroundings, in a priority of -1. Strictly speaking, this isn't a block update. However , there is a significant bug: When a block is placed/changed, if it changes again when self update, then the former change does not produce any updates that is in a priority of 0, 1, or 2. That's why self update is mentioned on this page.