What is this "forbidden" icon in my systray?

The following is a systray applet icon that appeared in my Unity session after I upgraded to Ubuntu 16.10. Its menu is tall, narrow and empty. What is it?

Solution 1:

The "forbidden" icon indicates that one of the panel indicator's (for example, Klipper) can't be loaded or is no longer functioning, possibly because it has been discontinued in Ubuntu 16.10.

If it hasn't been discontinued in Ubuntu 16.10, showing your system's process list by running ps -ef | grep indicator- will provide more information about which process is causing it. This will show the panel indicators that have the string indicator- in their names as the panel indicators from the default Ubuntu repositories do. To show all of the process list run ps -ef.