My IE9 is fine with trailing comma, user's IE9 is not; why?

I have a web app with a JavaScript file using code like the following:

var foo = {a:'b',c:'d',e:'f',};

On my computer, IE9 (9.0.8112.16421) works correctly; the JavaScript is parsed and executed as on Chrome/FF/Safari. Another user claims to have the same version of IE, yet gets this error in the Developer Tools Console:

SCRIPT1028: Expected identifier, string or number 
blob.js?modified=1304026278, line 524 character 136

开发者_运维百科Column 136 is the trailing comma/closing brace for the object literal.

I've 'fixed' the code so the user should have no more problems, but I'm wondering:

What could make the same version of IE on two different computers treat the JavaScript differently?

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One of the reasons may be that user has pressed the "Compatibility View" button. The trailing comma causes syntax error in IE7 document mode.

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Its most likely due to a browser setting suppressing the error on your browser.