HTML5 offline gets annoying, and why not just completely turn it off when in development? There should be an option to turn it on, for basic testing, but otherwise it just gets in the way.
This can easily be done by returning a 404 on the request for the appcache.
Note: this is also how you can make an html5 offline app no longer an offline app, and instead like a regular website.
Wish I could work on a PR for this, but I know nothing about rack, ruby, and/or rails. Maybe in 6 months
If you want it disabled temporarily in development, I would just comment out the line in your routes.rb. If you want it permanently disabled in development, but still active in production, wrap it in if Rails.env.production?