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.
Environment
Windows 7, Java Version 7 update 3(build 1.7.0_03-b05O)
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.