X = An expression
@ISERROR returns 1 if X ``contains'' an error, otherwise it returns 0. X ``contains'' an error if it it would generate an error outside of the @ISERROR function, which means that the expression itself may generate an error, or it may reference a cell which has generated an error.
@ISERROR(1) = 0
@ISERROR(3/0) = 1
@ISERROR(@ISERROR(3/0)) = 0
@ISERROR(C1) = 0, where C1 = @PI