Once, I wrote some code that read similar to this:

module Stub
  class Applicable end

  class ImmediatelyApplicable < Applicable
  end

  class DelayedApplicable < Applicalbe
  end
end

As this were just stub-classes they were not used in any test. They pipeline succeeded and deployed the code to a staging environment. This resulted in an error and the service was down, albeit working perfectly on my machine!

[2022-03-04T12:34:56+00.00 app[worker.1]: pid=1 tid=2ks] WARN: NameError: uninitialized constant Stub::Applicalbe

Turns out, I made a typo and wrote Applicalbe. *facepalm*

When in development mode, files are only required when they are needed the first time but when running Rails in production mode, it was eager loading all the files.

Turns out, you can reproduce this locally in your console to re-create the error:

$ bin/rails console
irb(main):001:0> Rails.application.eager_load!