Sticky pistons that are pushing or pulling a block during a frame rate spike appear to be duplicating the block it is pushing or pulling, but only if the player is either not looking at it or is outside a certain range (but not far enough to unload the chunks).

I've included photos of the result of this glitch, but it's a little difficult to show you what's going on with just photos, which is why I recorded the glitch in action as well: https://www.youtube.com/watch?v=51OoLW9ddy4&feature=youtu.be