Users that are slow to adopt newer browsers tend to make development much harder. By giving a somewhat familiar "call to action", users may be (in some small way) given incentive to upgrade.

This simple script creates a new element and allows it to be styled near to the default notice for IE.

All flavors of Internet Explorer are sniffed. Click the red box - which may be styled however you wish - to close the nag bar. Cookies can be used to prevent them from appear each time.

Newer Opera versions identify correctly, making that "false positive" still applicable for a browser upgrade.