Browser detection with Microsoft AJAX Library
Opera can disguise itself as a MS IE or Mozilla browser (Tools > Preferences | Advanced | Network > Browser Identification). This feature lets Opera "slip through" cases where access to a website is restricted to IE or Mozilla browsers. I noticed that while Peter-Paul Koch's browser detection script correctly identifies Opera even if a fake browser id is set, it is misreported by the Microsoft AJAX Client Library due to the way the browser detection code is implemented in MicrosoftAjax.js . Related: HOW TO precisely detect the browser version for IE 5 or higher