When you place a block diagonal to a dispenser/dropper, power that block, and then update the dispenser/dropper, it causes it to fire. To repeat the BUD again, you have to update the dispenser/dropper after you un-power the diagonal block.

Steps To Reproduce:
1. Place a dispenser/dropper
2. Place a block diagonal to that dispenser/dropper in any direction
3. Power that block with a repeater or redstone dust
4. Update the dispenser/dropper
5. Watch as it fires
6. Remove the power source
7. Update the dispenser/dropper
8. Repeat