Minecraft wiki entry for Quasi-connectivity, associated text: Where quasi-connectivity gets complicated is that it can cause states where a piston should be activated by QC … but doesn't know it. When redstone components change their state, they update other redstone component around them of the change so that they can update their state in response (for example, when a lever turns on, it updates nearby components that they should now be powered or activated). However, redstone components only update other blocks a maximum of two spaces away, but quasi-connectivity can create situations where a piston should be activated from a redstone component three spaces away. For example, a redstone component powering a block next to the space above a piston—the redstone component can activate the piston by QC but is three spaces away so does not provide an update to the piston.