Piston duplicates TNT block (and ignites it), if the TNT is being pushed and tried ignites simultaneously.

I made this video showing what I did: http://youtu.be/RoIwlEevT-Q

I found this weird bug with accident. Further testing is needed to fully understand what's going on. I haven't tested if this bug existed in previous version of Minecraft, but I'm pretty sure it's new.