Put wall.

A rack of small compartments, each one assigned to a single open order for as long as that order takes to complete. A picker who has just batch-picked forty items covering thirty orders scans them one at a time, the system names a cubby, the item goes in, and on the far side a packer pulls the cubbies that have gone complete. It is what makes batch picking usable: without a wall, one pass through the aisles produces a tote of mixed items and no finished orders. Walls are sized by the number of orders open at once and by arm's reach, so an operator should not have to take more than two or three steps, and a shop that batches wider than its cubby count stalls mid-wave with nowhere to put the next item. The second failure is quieter: a mis-sort puts a correct item into the wrong order, which reaches the customer instead of stopping the line, so it never shows up in a throughput number.