If a piece of redstone dust is being powered directly by some sort of power source (torches, levers, blocks, etc.), it cannot power any other blocks even if it leads directly in to them.

In attachment PEa, a line of redstone is being powered by a block placed next to the left redstone dust. The left torch doesn't turn off, despite there being redstone leading directly into the block it's attached to; however, the other torch on the right does turn off. In attachment PCa is the same circuit in PC, notice how both torches turn off.

In the attachment PEb, the same circuit is being powered by a redstone torch from above. Once again, the torch on the left where the redstone is being powered doesn't turn off, where the torch on the right does. Attachment PCb shows the same circuit in PC for reference.

In attachment PEc1 and PEc2 is a common half-adder design. The three torches on the top should be turned off since the redstone dust next to them is powered; however, the torches are still on. PCc1 and PCc2 show the same half-adder design, but the three redstone torches on the top are off.