The Illinois Undervote Notification Law, part of the 2007 omnibus election bill, required that by the year 2010 all Illinois voting systems detect and notify voters if they undervote, which is a ballot that does not include votes for one or more races.
In Illinois, the voter must be notified if they do not cast a vote for a statewide constitutional office (Governor, Attorney General, Secretary of State, Comptroller, and Treasurer). Depending upon the voting machine, some state and local undervoted ballots may also be rejected.
What this means is that if you don't cast a vote for every state race, your ballot will be refused. You will have to tell an election judge that you intended not to vote for every office and he will need to override the ballot, which may involve unlocking the machine and pressing an over-ride button several times befor it is accepted. Also, during that process, the election judge could be privy to those office races in which you did not vote.
So much for casting a secret ballot. And be prepared to stand in line because it is estimated that 60-70% of ballots in some counties are undervoted.