On Event Registration Form (classic) user experience an 'Unable to load data list' error' when there is something in the cart and the same person tries to register again. If they register as a new individual it works without error.

The hope is that Blackbaud can finesse the UI in this area as far as out of the box event registration parts are concerned as it becomes more common for users to already be logged into a session when they are reviewing and potentially registering for events.