If you pass an html element reference, then context is ignored.

Suppose you have this html:

<div id="my-div">...</div>

and this div is the target of a click event.

var myDiv = ev.target;

$(myDiv, $('#possible-containing-div'));

can give a different result than

$('#my-div', $('#possible-containing-div');

It's because of this if statement: http:github.com/jquery/jquery/blob/master/src/core.js#L59

Here's my use case that led to this discovery: (It failed b/c context was ignored.)

$(document.body).click(function (ev) {

if ($(ev.target,$('#container')).length) return;

// code to execute if target element was NOT in container

});