When a comparator takes the Input of a container, but between is a solid block, it will not care about the signal that is given by the block.

Screenshots 1 shows normal behaviour.

Screenshots 2 shows the exact same circuit, but an empty hopper is placed behind the block.

In scrennshot 3 I placed 1 item into the hopper
=> the comparator recieves only the signal from the hopper, not from the block

This video shows the bug too:

http://youtu.be/GEOt3K2XznA

I don't know if that is intended, but I've expected, that the comparator will recieve from both, the block and the hopper, a signal.