Sorry, this site hasn't been shared with you.

Just a page with this rather curt message and nothing else. No ribbon; no user welcome menu thingy; nothing. Hmmm.

It can be a bit fraught, especially when testing access permissions by switching between various users​. You might well see this if you try to log in as a regular user to a publishing site and you have content that hasn't been published (i.e. there is only a draft copy, which is visible to a user with edit permissions). This can also happen if there is a page layout or just some CSS or JavaScript that isn't published, or of course a master page. The problem is you are now rather stuck, because you don't have that handy "Log in as another user" menu item to work with. Or maybe you can see the page with the user welcome menu, but the "Log in as another user" option just isn't there. Restarting the browser often doesn't solve the problem because the browser has cached the credentials. How on earth are you supposed to sign back in with the right credentials?

Answer: use the URL http://site.domain.com/_layouts/closeConnection.aspx?loginasanotheruser=true. This should get you to the login prompt and you can then change user. No need to restart the browser or fiddle with the stored credentials settings in Control Panel.

Just one caveat: this option uses an unsupported browser feature which is potentially unreliable and apparently does not work in IE 10 and Safari. In this case an alternative is to start the browser with different credentials using the "Run as different user" context menu option which you can see if you hold down the shift key while right clicking the browser link.​ Another option is to find your credentials cache through control panel and remove the item.