Do you use Google Gmail? Have you tried the Gmail labs features? If you have and enabled the calendar feature of Gmail labs you may be seeing an authentication request if you are accessing Gmail using Google Chrome. For some reason Chrome cannot keep Gmail convinced that it is authentic in order to continue to display your Google calendar on the side of Gmail. Not sure why this is happening but if you do not want to continue to have to cancel this request you can disable your choice for the calendar in Gmail labs section.