Java 1.7 latest, Intel i7 4 core, windows 8, 8 gigs of ram.
Piston bug. Rather than documenting it, I hope you don't mind a 55 second youtube video. https://www.youtube.com/watch?v=V1wnXWGwcc4
I got another example. It happens on both 1 and 0 tick delays, it seems.
Sorry if this is a clone, just there are so many issues out there it's nearly impossible to know.
MC-1230 Players and mobs can fall through extending/retracting pistons