Repeaters pointed into a glass/air block above a dispenser powers the dispenser.

The repeater in the sample image is pointed into the air block above the dispenser so powering the repeater should not be powering the dispenser but it is. All 6 orientations of the dispenser are affected but only the air block above the dispenser can "power" the dispenser via a repeater. Replacing the air block in the sample image with any redstone non-conductive block or another repeater still sets off the dispenser.