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.

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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 => '...'