Minecraft wiki entry for Redstone_Dust, associated text: When redstone wire is reconfigured after placement, it does not update other redstone components around it of the change unless that reconfiguration also includes a change in power level or another component provides an update. This can create situations where a mechanism component remains activated when it shouldn't, or vice versa, until it receives an update from something else – a "feature" of redstone wire that can be used to make a block update detector.