SO invoice entry creating random backorders

Our invoice clerk report 3 times that she had back orders on invoice she had entered last week. We don't do backorders in our business so this is very unusual. The Backorder column is not on the SO Invoice Data Entry form.  The system seems to grab the unit price and put it in the backorder qty field automatically.

Has anyone else seen this problem and have a fix for it?

This is seriously slowing down invoice processing.