It could happen that the batch level for an item has been set incorrectly to zero when in fact this batch does contain a stock level. If that happens when the batch has already been assigned to an order, Linnworks will still allow you to scan this batch even though that action will result in a negative level for the batch. 

• An order contains batched items (SKU 100101-01-L) and the packing list has been printed.
  
  
• The level for BATCH-3 is set to 0
  
  

• As per the FIFO logic, item 100101-01-L from BATCH-3 has been assigned to the order to be picked and packed. The warehouse staff will pack 10 items from BATCH-3 even though the level has been set to 0 for this batch in Linnworks.
  
  
• -10 will be shown as the level for BATCH-3 for item 100101-01-L as a result of processing the order.
  
  

For details on how batch assignment works in the new Open Orders screen, please see this article.