One thing I wanted to do this time that I'd never done before was automate marking invalid form fields. I'd done this in the past using manual template changes, but that's a lot of grunt work.
Instead, my coworker Perrin Harkins suggested we use HTML::FillInForm to do the job. We're already using it to re-fill forms with errors, so why not use it to setup a CSS class on invalid fields too. It turns out HTML::FillInForm has a very similar facility already to disable selected fields, so adding an invalid fields feature was pretty simple.
I've already sent along the patch to the maintainer, but if you want to get it now here it is: