A vertical dropper will not activate when powered if another block two blocks above the dropper is simultaneously powered and a redstone wire in a certain location is pointing in a certain direction. This bug is reliably reproducible. It ruins some redstone creations.

In the image, when the blue wool is powered, the dropper and the red wool are simultaneously powered. If the red wool is powered and the redstone on top of the green wool isn't pointed into the blue wool (including if it's an unconnected dot), the dropper won't activate. Even if the redstone is facing into the blue wool, the dropper still won't activate if the wire is facing certain directions. I haven't figured out a pattern for this yet.