I've encountered a problem using custom Data Validations with XSSF sheets. Adding the validation seems to work OK, but afterwards when I try getValidations I get an XmlValueOutOfRangeException (full stack trace below). The contents of the validation formula doesn't seem to matter, and things seem to work correctly if I load a xlsx workbook containing custom validations and then try to access them. Here is some simplified example code that demonstrates the problem:

        XSSFWorkbook wb = new XSSFWorkbook();
        XSSFSheet sheet = wb.createSheet();
        sheet.getDataValidations();    //<-- works

        //create the cell that will have the validation applied
        sheet.createRow(0).createCell(0);

        DataValidationHelper dataValidationHelper = sheet.getDataValidationHelper();
        DataValidationConstraint constraint = dataValidationHelper.createCustomConstraint("SUM($A$1:$A$1) <= 3500");
        CellRangeAddressList addressList = new CellRangeAddressList(0, 0, 0, 0);
        DataValidation validation = dataValidationHelper.createValidation(constraint, addressList);
        sheet.addValidationData(validation);

        sheet.getDataValidations();    //<-- raised XmlValueOutOfRangeException

I also have the same problem when operating on an exisiting workbook that contains sheets and cells, I've just simplified the code above.