If your IP number changes during a login session (for instance when you are logged in on your laptop in one location and then move to another when using DHCP) you get the below error message:

Sorry, but your IP number seems to have changed from when you first logged in. This security feature prevents crackers stealing your identity while logged in to this site. Normal users should not be seeing this message - please ask the site administrator for help.

This message freezes non-technical users in their tracks--the message should contain a link to the login page so they can re-login and get a new session.