Piston pulse without any direct power

A piston pulses without direct signal.
There's a wire that goes toward a piston facing up, the wire is diagonally placed and interrupted by a block.
When i power the far side of the wire, the piston should not do anything, but it pulses.
I still have not figured out WHEN and WHY it happens, because most of the times the piston doesn't pulse.
When I found a position in which it happens, the piston pulses every time I trigger the wire, until I restart Minecraft.

How to replicate:
-place a block, redstone on it, another block above
-place a block on the side, redstone on it (the block above should interrupt the wire)
-piston attached to the last redstone dust.
-lever on the first block placed

Without the upper redstone, this doesn't happen. So that's not a BUD.