See PHI1963, where the billing system generated an invoice incorrectly. I believe the timeline here is:

• A customer created a support pact and an invoice was generated.
• Before they paid, the pact was credited for a long period of time.
• After the credit elapsed, a second invoice was generated, leaving the pact with two outstanding invoices. This is never a correct state for a pact.

This didn't actually apply any incorrect charges and step (2) requires abnormal administrator intervention.

In the case of pacts, the cleaner resolution would be to prompt-to-confirm and void outstanding invoices when a pact is credited. However, stronger general safeguards against ever having two outstanding invoices on a pact might also be reasonable.