You can see it on your online demo here at http://zssdemo.zkoss.org/zssdemo/excel_like
Click on H1 and type =( then click on cell G12 then press + then click on cell G13, then press ) then press / then click on G14.
You will see the warn box stating "
This formula contains an error : The specified formula '(G12+G13)/' contains an error. Expects cell reference or constant literal."
Click OK, then it works