Redstone lamp power is inconsistent.
To reproduce:
place redstone lamps & power them with redstone torches underneath
some will stay unpowered within loaded chunks until excessively close
Environment
Windows 7 x64
Java 7 Update 15 (64 bit)
Intel i7 3770
16 GB Ram
Radeon 6970
Duplicate of MC-1018. The bug is not that the redstone lamps are not lit, but rather there is a lighting glitch (which can happen without lamps) that causes black spots.