Status of Bill/Invoice changed from Paid in Full to Open

This could happen when the applying payment is a journal entry. Editing the account of journal entries that are applied as payments to any transactions will throw the following message:
 

"This journal is paying other transactions. Editing the journal will cause these payments to be unapplied. Are you sure you want to edit it?"

 

When you click on OK, the journal entry will be unapplied to any bills/invoices. Apart from the change of status, nothing else will be listed under the bill/invoice's System Note.