When the world unloads, often comparators pointing away from containers will continue to output their signal, even when the container becomes empty.
Steps to recreate:
1. Place a hopper 1 block above the ground.
2. Situate a comparator pointing away from the hopper.
3. Place a powered rail under the hopper.
4. Use redstone with an inverter to cause the hopper to activate the powered rail once the hopper is empty using the comparator as input.
5. Fill the hopper so that the powered rail is inactive.
6. Log out of the SMP server.
7. Log back in after 10 minutes (not sure if the duration matter).
8. Place a storage minecart under the hopper.
When the hopper empties, one would expect the powered rail to become active, causing the minecart to move away. Instead, due to this bug, this cannot happen unless the comparator is broken and put back. If it is left alone, it stays powered, even when the hopper becomes empty.
Possible Clues: This behavior was observed on SMP; player located greater than 1 million blocks away from spawn.