If multiple users are using the same client PC, you can run into following issue: when a new user logs in using the Sign in as Different User function, the user receives the error Access Denied. This error is thrown because the user is redirected by the Sign in as Different User function to the page which was last opened by the previous user and the new user doesn't has rights on this page. This can be confusing for end-users.

You can adapt the INIT.JS file in the 12\TEMPLATE\LAYOUTS\<lcid> hive as such that upon execution of the Sign in as Different User function, the page being displayed is the home page of the portal.

Look for the LoginAsAnother function in the INIT.JS file and adapt it as follows:

function LoginAsAnother(url, bUseSource){

document.cookie="loginAsDifferentAttemptCount=0";
if (bUseSource=="1") {

GoToPage(url);

}
else {

var ch=url.indexOf("?") >=0 ? "&" : "?";
// url+=ch+"Source="+escapeProperly(window.location.href);
url+=ch+"Source=/default.aspx";

}

}