Lines will be parsed by the templating engine even if they are commented out. The following configuration snippet will lead to parsing errors since the "<%= ... %>" marker will trigger variable substitution:

Configuration: 

test:
  clients:
    default:
      # uri: mongodb://user:password@mongodb.domain.com:27017/<%=  database_name || app_name %>_development

Log:

rails_1 | NameError: undefined local variable or method `database_name' for main:Object
rails_1 | (erb):19:in `<main>'
rails_1 | /home/integration-dashboard/.gem/gems/mongoid-6.4.4/lib/mongoid/config/environment.rb:40:in `load_yaml'
rails_1 | /home/integration-dashboard/.gem/gems/mongoid-6.4.4/lib/mongoid/config.rb:86:in `load!'
rails_1 | /home/integration-dashboard/.gem/gems/mongoid-6.4.4/lib/mongoid.rb:104:in `load!'
rails_1 | /home/integration-dashboard/.gem/gems/mongoid-6.4.4/lib/mongoid/railtie.rb:58:in `block in <class:Railtie>'
rails_1 | /home/integration-dashboard/.gem/gems/railties-5.2.4.2/lib/rails/initializable.rb:32:in `instance_exec'
rails_1 | /home/integration-dashboard/.gem/gems/railties-5.2.4.2/lib/rails/initializable.rb:32:in `run'
rails_1 | /home/integration-dashboard/.gem/gems/railties-5.2.4.2/lib/rails/initializable.rb:61:in `block in run_initializers'
rails_1 | /home/integration-dashboard/.gem/gems/railties-5.2.4.2/lib/rails/initializable.rb:60:in `run_initializers'
rails_1 | /home/integration-dashboard/.gem/gems/railties-5.2.4.2/lib/rails/application.rb:361:in `initialize!'
rails_1 | /opt/integration-dashboard/config/environment.rb:5:in `<top (required)>'