Exception while updating neighbors

I just built a 7 segment display following this tutorial: http://www.youtube.com/watch?v=GH7NbBKemSo

When I attach levers to the inputs, I trigger a lever, turn it off, and after a few seconds, Minecraft crashes. I tried restarting Minecraft and it just happened again.

What I expected to happen was...:
The pistons in the display should have extended.

What actually happened was...:
Minecraft crashed.

Steps to Reproduce:
I'm not sure of what caused the bug. Maybe the YouTube link could help.