Hi,
I observed this. When you disable form assistant for an entity and publish it and when you try to click on the follow up button on the form for that particular entity, it displays "Error on page" message.
Any one using the latest Rollup (3 or 4), kindly try this out and confirm is this error still persisting or Microsoft has taken care of it in the rollups.
Thank you.
Best Regards,
Gagandeep Singh
http://mscrmnovice.blogspot.com