Very odd posting error (ship and invoice a sales order)

We have a customer using nav 4/SQL who is getting a very odd posting error under fairly specific circumstances.
The error is:

The Reservation Entry already exists

This involves posting a sales order for an inventory item with item tracking and the serial numbers have been assigned already. They did not throw the error about serial numbers being required.

Now custom code is involved(pasted below), but the custom code does not reference the reservation entry table. I thnik that my problem is occuring because I am calling the posting codeunit from C/AL code rather than the magical way base Navision calls it via property settings on the menu item. I would like to think that Navision is setting a property or otherwise initializing something that I am unaware of.

BUT, the customer does not get the error when the invoice is posted from a superuser account, only from a "peon" account. This is the thing which just blows my mind.

Can anyone provide any insight?
thanks

=====================================

// mod to add qty validation before posting
   AllowToPost := TRUE;
   txtConfirmQty :='';
   KTICOde := '';

   TmpDetail.INIT; //tmpdetail is record var for Sales Line
   TmpDetail.SETFILTER("Document Type", 'Order');
   TmpDetail.SETFILTER("Document No.", Rec."No.");
   TmpDetail.SETFILTER(Type, 'Item');
   IF TmpDetail.FIND('-') THEN
   REPEAT
     IF NOT TmpDetail.Nonstock THEN BEGIN
       ItemLedger.INIT; // record var for Item Ledger Entry
       ItemLedger.SETCURRENTKEY("Item No.");
       ItemLedger.SETRANGE("Item No.", TmpDetail."No.");
       ItemLedger.SETRANGE(ItemLedger."Location Code", TmpDetail."Location Code");
       ItemLedger.CALCSUMS(Quantity);

       QtyAtLoc := ItemLedger.Quantity;
       IF QtyAtLoc < TmpDetail."Qty. to Ship" THEN BEGIN
          IF STRLEN(txtConfirmQty) < 180 THEN BEGIN
             txtConfirmQty := txtConfirmQty + 'Item No. ' + FORMAT(TmpDetail."No.", 20);
             txtConfirmQty := txtConfirmQty +   ' qty to ship = ' + FORMAT(TmpDetail."Qty. to Ship",7);
             txtConfirmQty := txtConfirmQty + ' qty-on-hand = '  + FORMAT(QtyAtLoc,8) +  '\';
          END ELSE
             txtConfirmQty := txtConfirmQty + '*\';

       END;
     END;
   UNTIL TmpDetail.NEXT = 0;

   IF txtConfirmQty <> '' THEN BEGIN
      txtConfirmQty := txtConfirmQty + '\ Post Anyways?';
      AllowToPost := CONFIRM(txtConfirmQty, TRUE);
      IF AllowToPost THEN BEGIN
         GetPassCode.OPEN('Enter code to force posting #1#', KTICode);
         GetPassCode.INPUT(1,KTICOde);
         GetPassCode.CLOSE;
         IF KTICode <> 'KTI' THEN
            AllowToPost := FALSE;
      END ;
   END;


IF AllowToPost THEN
   PostYN.RUN(Rec)
ELSE
   CurrForm.UPDATE(FALSE);