If you have a hopper inserting into a trapped chest with a normal chest to the North or West of the trapped one, the hopper will instead insert into the normal chest. It doesn't matter what direction the hopper is inserting from.
Environment
java version "1.7.0_09"
Java(TM) SE Runtime Environment (build 1.7.0_09-b05)
Java HotSpot(TM) 64-Bit Server VM (build 23.5-b02, mixed mode)
Mac OS 10.8.2
Duplicate of MC-7108, please use the search function to see if your bug has already been submitted. Currently over 42% of tickets are being closed as duplicate.