Please make allowances for my poor English.
When one strongly powered opaque block sends power simultaneously to the upper and lower adjacent redstone wire, the dropper or dispenser adjacent to the lower responds to the first pulse signal, but not to the second and subsequent pulse signals.
This happens only when the opaque blocks are at specific coordinates.
The coordinates seem to be distributed every 16 blocks on each of the xyz axes, and they appear to be axisymmetrically distributed around z=0 or z=-1.
I have confirmed that this bug has been occurring since 14w02a

reproduction video (YouTube)