The following code crashes the server when used in a listener.

http://pastebin.com/E2j9TU5L

It happens 100% of the time if you place a hopper directly under a chest, if you place the hopper first then a chest over it, it appears to function as expected without crash.

Code above checks to see if the inventory from is a chest or double chest, and the inventory to is a hopper, if so the event is canceled and the hopper is broken.

Stack Trace + Crash Report

http://pastebin.com/jBAsMcPX