If the Ruby on Rails mongrel server starts up in development mode but not in production mode, it could be because classes are only loaded when required in development mode, but they are all loaded up front in production mode. So if a class is missing, this will show up straight away in production mode.

As an example, I have an older version of rails running on the server (production mode) than on my laptop (development mode).
The older version of rails requires the application controller to be called
    app/controllers/application.rb
whereas the newer version requires it to be called
    app/controllers/application_controller.rb

In production mode on the server, mongrel was looking for application.rb and couldn’t find it, so fell over immediately. In development mode it started, but would have fallen over later.