Can’t sign in as a different user in SharePoint 2013

This Microsoft Knowledgebase article explains how you can sign in to Internet Explorer as a different user to get round this omission: http://support.microsoft.com/kb/2752600

Extract here:

1. Start Internet Explorer by using the Run as different user option, and then go to the SharePoint site.

Note: The Run as different user option is visible if you hold the Shift key when you right-click a program icon.

2. Access the SharePoint site by using the URL as http://siteurl/_layouts/closeConnection.aspx?loginasanotheruser=true

Important:

This option uses an unsupported browser feature which is unreliable and causes other issues. Currently this option does not work in IE 10 and Safari.