Button on left will correctly light up lamp on right with one configuration (Redstone_B) but not the other (Redstone_A), despite the only change being a single unconnected piece of redstone (on top of the cobblestone block).

Steps to reproduce:
1. Build the construction exactly like in the pictures without the restone on top of the cobblestone block.
2. Press the button and observe the lamp lighting up.
3. Add redstone dust on top of the cobblestone block and press the button.The signal will now not reach the lamp.

The bug is dependent on the pulse: changing the delay on the first (from left) redstone repeater from three to four will light the lamp. Replacing the second repeater with redstone dust or changing its delay will aslo let the signal pass. The behaviour works in all directions.