If you're experiencing a drop in FPS, even for a short amount of time, you may glitch through pistons if you're standing on them. Here's a build that might demonstrate this: https://www.youtube.com/watch?v=NZbJ-ldIgMQ

This glitch won't happen if you aren't lagging.

I also believe that lag may cause the pistons to behave abnormally, like not extending or extending at the wrong time.