Redstone dust can not be placed in the same location a light block is placed. 

I have a room full of light blocks to make sure the room is always lit. Recently i tryed to do some redstone and found i couldent place the redstone dust. I placed a block where i was trying to place a dust and the block would place. After i removed the block i could place the dust but only in that spot. Played around with this a little more and i found it was becues of the light blocks.