Command block minecarts make it possible to have an 'invisible' pulse.

Steps to Reproduce:
1. Make a command block minecart over an activator rail, and make it /setblock a redstone block near it.
2. Put an activator rail with command block minecart next to the redstone block. Make it /setblock an air block in the position of the redstone block.
3. The redstone block will look like it's disappeared. Place a piece of redstone dust next to where the redstone block was. It will not appear to pulse, but it will still power dispensers, etc.

EDIT: This might be a directional bug.