Piston extension bug

I made an automatic sugar cane farm, and this was before I pulled the lever:
http://imageshack.us/a/img824/5951/20130213181119.png
And after that, this happened:
http://img442.imageshack.us/img442/293/20130213181154.png
This only happened when the sugar canes grew next to the top piston. I also tried without any sugar cane growing in it, then it worked fine. I was playing on a server when it happened.