You've been there: A form cannot be submitted, but you don't see a validation error because the field at fault has no corresponding input field on the form. Because this is usually a bug, you insert debug information listing all errors into the form view. And once the bug is fixed, you forget to take out that debug information.
There is a better way. By copying one of the attached initializers into config/initializers, your forms will always render a small box listing all form errors in the bottom right corner of the screen. This box is never shown in environments other than development.
For older Ruby versions, use form_for_with_development_errors.rb. This monkey patches only form_for. For newer Ruby versions, use form_with_development_errors.rb. This monkey patches both form_for and form_with.
Rails-XSS is required by default. So if your project doesn't support Rails-XSS just remove all .html_safe calls.
Example
