The pistons, next to a slab with a redstone powder above, can be normally activated only when the redstone powder is in the direction of the piston. But on the screenshot, we can see that in an event (in the right top corner), the piston is activated despite the redstone powder is not in the direction of the piston graphically.

So, the game don t consider the new direction of the redstone powder when the redstone powder, which give this new direction (the last powder after the torch), is higher than it and is on a slab.

PS: Sorry for my bad english.