When you place redstone wire on a block of glowstone and connect it downwards to a glowstone block next under the other, then it results in the lower redstone wire note being powered.
The same problem occurs when using half slabs.
MC-6723
Redstone bug
Resolved
Duplicate
0
maikel van der knaap
(Unassigned)
Unconfirmed
redstone
Snapshot 13w01b
Retrieved
This is not a bug, but a feature. You will notice that the redstone can be powered upward, but not downward on these blocks. This can be useful in creating certain circuits.