This can happen for a number of reasons including but not limited to:
  • Bug 60533 - source data link gone for registrations with no reg fee but with an upsel: If you have a participant select a zero dollar registration type who also selects an upsell, this will happen.
  • Bug 67565 - Donations 2 Allows multiple transaction processing in registrations processing: This happens when a participant registers through an event that uses donations 2 and clicks the register button twice. This appears to be fixed for new events but old events and copies of old events can still have this problem.
  • The transaction become stuck in an error on the back-end.
In all of these cases, the solution to correct this is to contact support.