It is likely that what you are seeing is an error. I have only seen one other report of this issue, so it would be a great help if you can email us at email@example.com with any information that you can. Here are some specifics that will help greatly:
Username of affected Constant Contact account
Date/Time that the error occurred
Name of the affected email campaign
Name of any other affected email campaigns
We currently have our team investigating this issue so that we will be able to work towards a resolution. If you have any questions, please let me know!