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

Question

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

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?

Answer 1

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