My custom list item form (custom ASP.NET page) for a new item wouldn't appear.
When I went on to create a new item, I got the standard SharePoint page and not my custom ASP.NET page.
I was setting up a Feature with a list template. I had a schema.xml file and a NewProductForm.aspx generated from SharePoint Designer. I put NewProductForm.aspx in the same directory of schema.xml and changed the Forms section in schema.xml to 'register' the new form.
Removing the SetupPath attribute from the Form element solved the problem.
What led me to the fix:
I said 'abracadabra', threw salt over my left shoulder, read about How SharePoint Forms Work and this article about Custom List Feature Definition.