• Rack has a limit for how many form parameters it will parse.
• This limit is 65536 by default.
• There is a bug in Rack that will incorrectly count the number of input fields in nested forms. In my case a form with 1326 input fields was enough to break the default limit.
• If Rack thinks your request is too large, the request will fail with a low-level Ruby message like Fix: "undefined method `bytesize' for #" or the standard Rails error box.
• You will not get an exception notification per e-mail or Airbrake.

Why?

Rack has introduced this limit to prevent your server from being DOSed through large form submissions. The default value should also be enough for anyone if Rack counted nested params correctly.

Fixes

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

This seems to be fixed Show archive.org snapshot in newer versions of Rack, so you might be able to fix it by upgrading rack.

You can also increase the limit by a reasonable amount by adding this to your environment.rb, application.rb or an initializer:

if Rack::Utils.respond_to?("key_space_limit=")
  Rack::Utils.key_space_limit = 262144 # 4 times the default size
end