There has been a report of a donation that was successfully processed through PayPal, but recorded as "Abandoned at PayPal" in the control panel. Because the donation was not registered, this individual's event RSVP was not recorded.
This is something we'll consider looking into. We don't have a timeline for a fix yet but we will let you know when we do. Please email email@example.com with links if you see additional examples of this occurring.