When Rack::Bug has been added to your project and your Apache2/Passenger only replies with an Error 500 (Internal Server Error) you won't get any love from both application and Apache logs.
You can start a script/server and try connecting there. It should also fail but you will most likely see this error:
Internal Server Error
undefined method `new' for "Rack::Bug":String
While the following is (for some reason) working on OSX...
config.middleware.use "Rack::Bug", :secret_key => '...'
...you need to do this so it works properly on Linux:
require 'rack/bug'
config.middleware.use "Rack::Bug", :secret_key => '...'
Posted by Arne Hartherz to makandra dev (2011-12-05 15:27)