If when you start up Rails locally you see the message;
Then this means you probably have the Vanity Gem installed in your project.
This is an A/B testing gem, and the error is basically saying that in one of your experiments you have not specified a default. When this happens the first of the alternatives is picked.
For example; if you had the following in an experiement file;
I would expect you to get an error on startup of your Rails server saying;
In order to fix this we simply need to specify a