What does "An illegal choice has been detected. Please contact the site administrator." mean?

What does "An illegal choice has been detected. Please contact the site administrator." mean?

It means the web server didn't know how to deal with some of the data submitted.  We've found for CW2.0 this tends to happen when a user has multiple browser tabs or windows open to the site.  Some of the partially submitted data gets sent along when save or submit is clicked in the other tab/window.  Just close one of the tabs and try again.