This issue has been posted before, but it was 'resolved'...
Minecraft 1.5.1 still has this issue.
Hopper 1 has items. Hopper 2 (below the down facing hopper 1) has free space.
Hopper 1 is locked by redstone signal, hopper 2 is not. Hopper 2 pulls items from hopper 1...
Is this really supposed to happen?
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Duplicate of MC-6883, please use the search function to see if your bug has already been submitted. Currently over 54% of tickets are being closed as duplicate.
That is intended, Powering a hopper only prevents that hopper from pulling and placing items. The lower hopper is not powered, so it can still pull items from the inventory above it regardless of it being another hopper, powered or otherwise.