In a setup shown in pressure-plate-on-a-fence.png, all of these 4 dust are powered when the pressure plate gets activated. This is strange because these dust aren't adjacent to the plate.

It seems the bug only affects redstone dust:

• Mechanical components in this position will not be activated.
• Solid opaque blocks in this position will not be powered, neither strongly nor weakly.

It also seems the bug can only be triggered by fences. Dust will not be powered if you replace the fence with an upside-down slab or stairs.