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.
I thought at first that it might be the redstone torches (top left of above pic) but they aren't powered. 
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?
Best Answer
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):
Here I updated the block by adding glass, then removed glass and power:
Here I removed all the remaining blocks and it still doesn't update. Then updated it with glass again:
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:
If you want to understand how it is being powered in this way, the wiki has a tutorial on quasi-connectivity.