When you power a piston with one redstone line, and depower it with another redstone line in the same redstone tick, a graphical glitch occurs for a split second. The piston block appears retracted, when it should be extended. To reproduce, build the redstone machine shown in the 2nd photo, and watch the piston. The piston can be normal or sticky. This bug was first introduced in 1.5.1, not counting snapshots.