One of the issues that can trip up new developers is the fact that forms require enctype="multipart/form-data" when there's a file being uploaded. If a developer doesn't know this, it can be hard to track down. I think it'd be reasonable to have the toolbar inspect the loaded HTML content and inspect it for invalid configurations. The first of which being a <input type="file" ..> /> in a form that isn't defined as <form enctype="multipart/form-data">.