A specific configuration of a comparator (in subtract mode) creates a rapid clock which is used to empty items from a connected dropper. In the attached image, there are two very similar circuits with the same components, with the comparator forming the clock in a slightly different position. The arrangement on the left will fire only a single item the first time the dropper receives power, and will not dispense any further items while the comparator clock is running. The configuration on the right works as expected, dispensing an item once per redstone tick. This does not appear to be caused by any known redstone behavior (i.e. quasi-connectivity), so I figure this is likely a bug of some sort.

There are no redstone components other than those explicitly and clearly visible in the attached image (no redstone torch hiding under either dropper, locking it in the "on" state, for example). This bug is identically repeatable in multiplayer and singleplayer, as well as in both creative and survival. The system depicted is not sensitive to orientation (north/east/south/west), and works the same way when mirrored.