We  have discovered Backorders being generated in a strange way on our system.  We are using Sage 100 2017 Advanced.

If we process an order in Sales Order Invoice without using a Sales Order (immediate sale).  We accept the invoice and then go back into it before posting, and lower the quantity on an item.  This will create a Sales Order - Backorder for that item when we post the invoice.  We have been able to duplicate this and don't understand why it should occur.  We do not receive any notice that a Sales Order - Backorder is being created.  It just does it.

Our current workaround is to delete the line and then add a new line with the correct quantity if we need to make a change.

Please try it yourself!

The Daily Back Order Report (which is printed with the Daily Sales Reports/Updates) shows the Backorder that will be generated.  If you are modifying an invoice, and you don't want a Backorder created, make sure to change both the Quantity Ordered and the Quantity Shipped fields.

The Daily Back Order Report (which is printed with the Daily Sales Reports/Updates) shows the Backorder that will be generated.  If you are modifying an invoice, and you don't want a Backorder created, make sure to change both the Quantity Ordered and the Quantity Shipped fields.

Did you actually try this?  When we did. It did not generate a Daily Back Order Report.  We did change both the Quantity Ordered and the Quantity Shipped.

Yes, I "actually" tried this, and it worked as I outlined above.  

That report is optional.  Look in SO options.

Back-orders should only be created if #Ordered does not equal #Shipped.

Did the order originate as a sales order or place initially as a sales order invoice.  This will occur if it is done as a sales order invoice.