class UsersController
  def create
    @user = User.new(params[:user])
    respond_to do |format]
      if @user.save ...
      else
        format.json ....
      end
  end
end

Error occurred while parsing request parameters.
Contents:

user: {login: "John", email: "john@yahoo.com", password: "111"}}
/! FAILSAFE /!  Mon Nov 08 02:01:04 -0800 2010

  Status: 500 Internal Server Error
  Invalid JSON string
    c:/ruby/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/json/backends/yaml.rb:14:in `decode'
    c:1:in `__send__'
    c:1:in `decode'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/params_parser.rb:42:in `parse_formatted_parameters'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/params_parser.rb:11:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/cookie_store.rb:93:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/failsafe.rb:26:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `synchronize'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:114:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/reloader.rb:34:in `run'
    c:/ruby/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:108:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/static.rb:31:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:46:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `each'
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `call'
    c:/ruby/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/log_tailer.rb:17:in `call'
    ...
    c:/ruby/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:34:in `run'
    c:/ruby/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:111
    c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
    c:/ruby/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
    script/server:3
re.rb:31:in `require'
    script/server:3

begin
  format.json
rescue ParseError => e
  render :text => "Now there's some ugly JSON! (#{e.message})", :status => 500
end

class UsersController < ApplicationController
...
  rescue_from ParseError do |e|
    render ...
  end
...
end

def decode(json)
  if json.respond_to?(:read)
    json = json.read
  end
  YAML.load(convert_json_to_yaml(json))
rescue ArgumentError
  raise ParseError, "Invalid JSON string"
end