Prior to Release 4.0, the "delivery completed" indicator for a purchase order and production order was automatically reset in the case of return deliveries or reversals if the issue posting led to a violation of the underdelivery tolerance limit. This could go unnoticed, as you did not receive a message.
As of Release 4.0A, you receive a warning message that the "delivery completed" indicator has been reset. After receiving this message, you can select the "delivery completed" indicator again if another delivery is not expected. The system no longer automatically changes the "delivery completed" indicator after you have set this manually.
For more information on underdelivery, overdelivery and final delivery, refer to the R/3 library - MM Materials Management - Inventory Management - Goods Receipts for Purchase Orders section - Underdeliveries, Overdeliveries, and Final Delivery.