Have you ever filled out a form only to have it rejected because of an unexpected, overly-restrictive validation error? It's frustrating and annoying, and it probably happens to your users too.
Guardrail shows you the validation errors your users are experiencing, giving you a new way to uncover UX issues in your app.
Guardrail works with ActiveRecord on Ruby on Rails 2.3.x and ActiveModel (including ActiveRecord, Mongoid, etc.) on Rails 3+.
Pretty amazing app if you want to know which form validations people are getting tripped up on. It's helped me to design my forms to have better hints and make overall UX a lot better for my users.