When you place a block next to a hopper, with the block on the EAST side i.e. the hopper is on the WEST side you get a lighting bug. The hoppers insides (only) are much darker then it is meant to be.
However if the block is on a side other then EAST the bug does not appear. See screenshot.
Note: The signs have no effect (I have checked this) they are there just to make the screenshot easy to understand.