Dropped items appear to get caught in the gap between fenceposts and flowing water next to them, and may teleport back and forth between two points.
Steps to reproduce:
- Create the device in "Flowing 1". There is one water source block in the lower-right corner, where the cursor is pointing.
- Throw an item into the water flow, so that it moves straight into the fence. (Flowing 2)
- Place a block in front of the water source block, forcing the water to flow around it. (Flowing 3)
- The item will float downstream, approximately to the point in "Flowing 4", and then teleport back to the fencepost.