This happened with revision 9148.
Using PayPal Standard.
Scenario, person registering for an event with a fee requirement.
fee clears Paypal and a completed contribution record is created.
No registration for the event is created.
fee clears Paypal and a PARTIAL contribution record is created but left at status pending.
Complete information from PayPal shows up in the CiviCRM.log file including a completed status.
The following error is in the CiviCRM.log file:
Apr 03 08:49:20 [info] Could not find an entry for contributionID in GET
Please let me know if I can provide any additional information.