When an input is disabled by its parent fieldset the :disabled selector behaves differently in IE ( any version ) then other browsers. In other browsers the :disbaled selector will indicated an input disabled by a parent fieldset, where in IE it will say it is not disabled.

http://jsbin.com/padopu/3/edit?html,js,output

My first thought was sizzle was falling back to QSA and the :disabled selector where available and this was not working correctly in IE, but QSA works correctly in IE9-Edge for this.

There seems to also be a related documentation issue as the docs say this always checks the disabled prop. see jquery/api.jquery.com#734