When a hopper has one or more items in any of slots #2-5 while slot #1 is empty, and then it receives an item into slot #1 (pushed in by another hopper or a dropper, or moved in by a player), it will try to push out that same item from slot #1 instantly. This violates the 8 game tick transfer cooldown that hoppers are supposed to have.

Impact

Several impacts of this behavior can be seen in a horizontal line of hoppers pushing in the same direction (known as a "hopper pipe"):

• Hopper pipes can be accelerated to transfer single items faster than 1 block/8 gameticks. This applies to both Java Edition and Bedrock Edition, as demonstrated by Ilmango and NavyNexus
• Items moving through a hopper pipe can skip past item filters built under the pipe. This only happens in Bedrock edition because it depends also on random update order (MCPE-28890). Bedrock players have to expand their storage builds with transfer loops, extra filters, or rate limiting redstone contraptions to overcome this. However, even looping can fail: see MCPE-166794.
• Items moving through a hopper pipe can skip past each other, rearranging their order. Again, this only happens in Bedrock edition because it depends also on random update order (MCPE-28890). Bedrock players have to expand their potion brewing builds, and will have to expand builds for the upcoming crafter block, by adding extra buffer space or redstone contraptions to control the sequence and/or limit the rate of item flow in hopper pipes.

Steps to reproduce

1. Load Hopper cooldown skipping.mcworld. This world contains two item filter test setups and two pairs of long hopper pipes, with books that explain the builds. These builds demonstrate the first two impacts described above.
2. Read the book by the item filters and perform the tests described in it.
3. Read the book by the long hopper pipes and try each pair of hopper pipes by pressing the button.
4. Load MCPE-28890 (crafter).mcworld. This world contains a contraption designed for the upcoming crafter block, which is meant to loads items into a chest in a specific order.
5. Press the button on each version of the build.

Expected results

In the first test world, the zombie heads would never skip past the item filters and the wired and loaded hopper pipes would have the same transfer rate as the empty hopper pipes. In the second test world, the items would always enter the chest in the same order in each build.

Observed results

In the first test world, the zombie heads skip past the item filters in predictable ratios depending on the setup. The wired and loaded hopper pipes transfer individual items much faster than the empty hopper pipe. In the second test world, in the smaller build the stick sometimes enters the hopper before the redstone.