jQuery does not select the same elements as querySelectorAll when given the same CSS selector string. Specifically, it does not handle the case where the selector contains a #id referring to an element above (or the root of) the sub-tree given as the context parameter of the jQuery search.

var w = document.createElement('div');
var x = document.createElement('div');
var y = document.createElement('div');
var z = document.createElement('div');
var match = x.mozMatchesSelector || x.webkitMatchesSelector || function() { return 'fail'; };

x.id = 'Hello';
x.appendChild(y);
z.className = 'blah';
y.appendChild(z);

console.log($('#Hello .blah', y).length); // prints '0'
console.log(y.querySelectorAll('#Hello .blah').length); // prints '1'
console.log($(z).is('#Hello .blah')); // prints 'true'
console.log(match.call(z, '#Hello .blah')); // prints 'true'

w.appendChild(x);
console.log($('#Hello .blah', x).length); // prints '0'
console.log(x.querySelectorAll('#Hello .blah').length); // prints '1'
console.log($('#Hello .blah', w).length); // prints '1'
console.log(w.querySelectorAll('#Hello .blah').length); // prints '1'

I especially want to emphasise that $(node).is(sel) says the selector DOES match and yet the normal jQuery(sel, node.parentNode) fails to do so.

(Confirmed in Firefox 14 and Chrome 20)