ContentDatabase is null
You've created a brand new rendering and it just won't show up. When you try to navigate to any content item which uses it, you receive the following error:
System.InvalidOperationException: The Sitecore.Client.ContentDatabase is null.
Be sure to publish the parent folder of the rendering. Sitecore will happily allow you to publish the rendering even if it's parent has not been fully published. This is often the case when a new rendering is created in a new folder.