Vasari writes that when Cimabue was away from the workshop , Giotto painted a fly on the face of the painting that his master was working on . When Cimabue came back , he tried several times to brush the fly off . He writes that when Cimabue was absent from the workshop , his young apprentice painted such a lifelike fly on the face of the painting that Cimabue was working on , that he tried several times to brush it off .