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I've lately become hooked into building computers in Minecraft and today I decided to build a giant hard drive. I've built HDDs before, but not at the scale I'm currently attempting, and I anticipated problems. As I was wiring some pistons, they activated and pushed forward oddly. As far as I can tell, there's nothing powering them.

enter image description here

I thought at first that it might be the redstone torches (top left of above pic) but they aren't powered. enter image description here

Here's a top view of the same pistons. I also thought the line of redstone above them might be the problem (middle of above pic), but when I destroy it, the pistons stay extended.

Is there a reason why my pistons are extending? If so, is there a way to get them to power (when I want them to) without disturbing the pistons above?

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You are experiencing quasi-connectivity. You have actually produced a BUD (Block update detector).

The power source is the redstone line above. I have built an example to demonstrate.

Here it is unpowered and then powered(but not updated): First

Here I updated the block by adding glass, then removed glass and power: Second

Here I removed all the remaining blocks and it still doesn't update. Then updated it with glass again: Third

Note: There is a sandstone block next to piston. I removed that when piston was powered. Unpowered would have updated piston and it would have retracted.

Unfortunately you will have to go with a different circuit design. One possible change would be to use direct power instead of inversion of the signal to invert all the torches. Run a line directly behind/next to pistons.

Here is an example: Example

If you want to understand how it is being powered in this way, the wiki has a tutorial on quasi-connectivity.

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This seems to be caused by the famous bug MC-108. If you turn off the redstone line, the pistons first stay extended, but when you then place a block next to one of them, the line retracts. That's because the pistons are powered if the block above them would be powered, but that information doesn't affect them until they get a so-called "block update", which is sent when placing, breaking or changing blocks next to them.

This is not a property of redstone itself, it only affects pistons, droppers and dispensers. It also does the opposite: If you power the block above a piston, it doesn't extend until you update it.

There are different opinions about this bug, some abuse it to make circuits compacter, but it's very confusing for new redstoners, for example you. Personally I find it sad that it is in the game so long already. Whenever it gets fixed in the future, people abusing it will get upset, because they rely on it.

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    IIRC the devs said years ago that they would never fix it because block update detectors were so useful for so many things. It stopped being a bug and started being a feature :)
    – Cheezey
    Jul 24, 2017 at 2:54
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    It was never 100% clear if that's really the case. The observers were added to make BUDs possible without abusing this bug and Mojang's opinion about this seems to vary from time to time and from person to person.
    – Fabian Röling
    Jul 24, 2017 at 5:38
  • @Fabian This may or may not be caused by being sold to Microsoft. In such a big company it probably isn't that easy to just say "It's a feature".
    – Roman Gräf
    Jul 24, 2017 at 6:50
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    @RomanGräf: I like bashing Microsoft as much as the next guy, but in this case they can't have anything to do with it. MC-108 was marked "working as intended" in July 2013, while Minecraft's sale was only announced in September 2014.
    – jazzpi
    Jul 24, 2017 at 10:31

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