This week's book giveaway is in the Testing forum. We're giving away four copies of The Way of the Web Tester: A Beginner's Guide to Automating Tests and have Jonathan Rasmusson on-line! See this thread for details.
There isn't any exception stack trace there. That Log message is just at the info level, not an error. All it is saying is that there is an object in the collection that has no id set, so therefore no record in the database to delete.