When actions are blocked by sandboxing flags, we should log to the console some descriptive warning. For instance, we should probably log something like "Blocked form submission to '[URL]' because the form's frame is sandboxed, and the 'allow-forms' flag is not set." (http://code.google.com/searchframe#OAMlx_jo-ck/src/third_party/WebKit/Source/WebCore/loader/FrameLoader.cpp&exact_package=chromium&q=SandboxForms&type=cs&l=344). This goes for most (all?) of the flags; I don't think we're currently logging for any.
This is a goodfirstbug kind of bug, and Markus expressed some interest this morning in taking it on. Handing it to him for the moment.