Activated hoppers are allowing items to pass through.

Steps to reproduce

The issue occurs under the following circumstances.

1. Place three hoppers in a vertical column (see first attached image).
2. Power the middle hopper with a redstone block.
3. Place items in the top hopper.
4. Observe that the items pass through the middle hopper to the bottom hopper.

Alternative ways to reproduce

The issue still occurs under the following circumstances:

• Placement of the hoppers on chunk boundaries or within a chunk.
• It is not sensitive to the type of item being transported.
• Middle hopper powered by a repeater or redstone torch.
• Middle hopper powered indirectly.
• Middle item replaced with a chest (see second attached image)
• Top hopper powered instead of the middle hopper.
• Middle hopper is powered in all four directions.

Does not occur

The issue does not occur under the following circumstances:

• Both top and middle hoppers powered
• If the hoppers were placed prior to 1.2.x