config.ru

use Rack::Logger
app = proc do |env|
  [ 200, {'Content-Type' => 'text/plain'}, "a" ]
end

run app

#This will produce: 
# Rack::Lint::LintError: rack.errors#close must not be called
	/opt/local/lib/ruby/gems/1.8/gems/rack-1.1.0/lib/rack/lint.rb:18:in `assert'
	/opt/local/lib/ruby/gems/1.8/gems/rack-1.1.0/lib/rack/lint.rb:417:in `close'
	/opt/local/lib/ruby/1.8/logger.rb:510:in `close'
	/opt/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
	/opt/local/lib/ruby/1.8/logger.rb:509:in `close'
	/opt/local/lib/ruby/1.8/logger.rb:418:in `close'
	/opt/local/lib/ruby/gems/1.8/gems/rack-1.1.0/lib/rack/logger.rb:17:in `call'
	/opt/local/lib/ruby/gems/1.8/gems/rack-1.1.0/lib/rack/lint.rb:47:in `_call'
	 ...