It's possible that claims stated in the jQuery dos are inconsistent, but usage of the :checked selector yields incorrect results if on the queried ''form'' element resides ''select'' element while documentation is saying that:

''The :checked selector works for checkboxes and radio buttons. For select elements, use the :selected selector.''

Working code snippet is here: http://jsfiddle.net/J7Quh/1/

This behaviour can be reproduced on versions 1.6.4 ... 1.8.2.