Consider the following scenario:
Note If you run Internet Explorer as another user by using the Run as a different user option, this issue does not occur.
            - You run Windows Internet Explorer 9 or a later version of Internet Explorer on a computer that is running Windows 7 or a later version of Windows in a Microsoft Lync Server 2013 environment.
 - You sign in to Lync Web App as a guest.
 - You click Join the meeting.
 - You install the ActiveX plugin for Lync Web App.
 
Note If you run Internet Explorer as another user by using the Run as a different user option, this issue does not occur.