Tripwire disconnecting when a block is placed under the tripwire hook.

When a block is placed under a tripwire hook in the south or west direction the tripwire disconnects from the hook when someone walks into the tripwire, but when there isn't a block under the tripwire hook it works properly.

Here is a video showing what happens.
http://www.youtube.com/watch?v=8XB6sZ27uJU