Forms 2.0 added the handy feature of being able to override the form for known leads, and give them a direct download button instead of needing to fill out a form again.
The problem, though, is that hidden form fields, namely those that set their value from URL parameter, don't get captured when you use this functionality. We use these URL parameters across our campaigns, and need leads to be updated with the most current information. (we have two sets of these fields, one for original source info that blocks field updates, and the other for most recent source, which can change all the time) Here's the scenario:
Your form has hidden values for utm_medium, utm_source, and utm_campaign. Any new leads who come through this form will have those values populated by the URL they came in from (eg utm_medium=cpc&utm_source=ggl&utm_campaign=branded+terms).
For existing leads who come to this form from a tagged URL (eg. utm_medium=email&utm_source=newsletter&utm_campaign=best+practices+whitepaper), they'll see the custom HTML you've set as "Welcome back {{lead.first name}}, with a direct download button to whatever asset you want them to look at. When the lead clicks the submit button, the activity log will show the form submission event, but Marketo won't capture those URL parameters in the hidden form fields as it would if they were actually filling out the form. This may be a feature limitation or actually a bug, but it hurts our ability to leverage the known lead form functionality.
This causes problems for situations where you want to track First Touch (FT) and Last Touch (LT) campaign activity, because your URL parameter fields will conceivably not get updated for known leads.
I'd like to be able to use the custom HTML option for known leads on my forms, and not lose the ability to capture hidden form field values.