When I try to compile jquery v1.6.1 (src) with google closure I get:

./jquery/jquery-1.6.1.js:2460: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

if ( parent ) {

^

./jquery/jquery-1.6.1.js:2461: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

parent.selectedIndex;

^

./jquery/jquery-1.6.1.js:2464: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

if ( parent.parentNode ) {

^

./jquery/jquery-1.6.1.js:2465: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

parent.parentNode.selectedIndex;

^

./jquery/jquery-1.6.1.js:4300: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

if ( elem.parentNode ) {

^

./jquery/jquery-1.6.1.js:4301: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

elem.parentNode.selectedIndex;

^

./jquery/jquery-1.6.1.js:4577: WARNING - Suspicious code. This code lacks side-effects. Is there a bug?

Array.prototype.slice.call( document.documentElement.childNodes, 0 )[0].nodeType;

You can get closure here: http://code.google.com/closure/compiler/

Its very easy to reproduce:

java -jar compiler.jar --js=./jquery/jquery-1.6.1.js --js_output_file=out.js