How to investigate:
1. Check if the item is set to inactive by navigating to Lists > Accounting > Items > Show Inactives = T.
2. Go to item record and in the System Information tab > System Notes sub tab > check the Date it was set to Inactive (from F to T).
3. Go to the Sales Order record where this item has been selected. Check the Date when it was created.
4. Compare the Dates in the two records and verify which created first.
Note: Only Sales Orders to be created after the date when the item was set to inactive will not be able to select the item.