For those of you who don't play Minecraft, redstone (not to be confused with the Windows 10 update that has the code name “Redstone”) is a special substance in the game's world that can transmit power. PC and Mac users also have another way to use redstone called quasi-connectivity, and it's actually a bug that, in Mojang's own words, has turned into a feature.
In the PC edition of Minecraft, pistons can receive redstone power from a distance of two blocks when they are placed in a very specific diagonal position, rather than from a block right next to it. But blocks only get updated when something affects adjacent blocks. It means that you can power a piston and then remove the power without the piston detecting it! It puts the piston in this state where it actually needs to be deactivated, but doesn't know yet. Clever minecrafters exploit that to build elaborate trigger mechanisms known as Block Update Detectors: when something happens to a block next to a piston – like a block being placed or destroyed, gravel falling, fire igniting, rails re-orientating or even cake being eaten – the piston will “wake up”, notice that it isn't powered any more, retract and trigger something else.
The Windows 10 and Pocket versions of Minecraft did not add this “feature” but Mojang decided to create an Observer block that basically does the same thing:
It checks for changes in the environment and, if triggered, switches between emitting a strong signal and an unpowered state – but without relying on a bug to do so. We've tested this with lots and lots of different builds, and we can't wait to see what you lot do with it. We've also added some other cool stuff: pistons can now push chests and many other things!
PC and Mac users will still have this redstone bug in place; Mojang does not intend to “fix” it.