After getting form through f = $('#someform'), or e.g,

<form onsubmit="foo(this)">

function foo(some_form) 
{
    var f = $(some_form);
    f.elements.each(.....) // f.elements undefined
}

its elements become unavailable: f.elements becomes undefined, as well as any previous members, e.g "f.bar" if there was an input name="bar".