When redstone torch is placed on certain vertical line, some weird behaviour is observed (the placement kind doesn't matter: on a wall, on the ground, side of the wall didn't seem to matter either).
The redstone line does power pistons below the blocks that redstone is placed on. But pistons are not de-powered (do not retract) when the redstone signal goes away. Seems to only happen in case of a straight redstone wire, and at limited length of redstone wire. The direction of pistons doesn't seem to matter.
There seem to be more than one such vertical lines in a chunk. These positions seem to be the same for every chunk (i.e., relative to chunk's lower x and z coordinates).
In the attached testing world provided, only one such vertical line is marked. I can search and find more later if needed. After, one can enumerate 256 of their positions in total...

P.S. The site doesn't allow to select it, but checking in other versions has shown this bug was present since 1.8. It was absent in 1.7.10.