If the formula contains a space or _ (underscore) charcters, setCellFormula 
throws array out of bounds exception.

Sample code:

import java.io.*;
import org.apache.poi.hssf.usermodel.*;
import org.apache.poi.hssf.*;

public class FormulaError
{
  public static void main(String args[])
  {
    try
    {
      // New workbook
      FileOutputStream out = new FileOutputStream
("D://java//programs//test.xls");
      HSSFWorkbook book = new HSSFWorkbook();
      HSSFSheet sheet1 = book.createSheet("Test Sheet1"); // Note the space in 
between
      HSSFSheet sheet2 = book.createSheet("Sheet2");

      HSSFRow row = sheet1.createRow((short)0);
      HSSFCell cell = row.createCell((short)0);
      cell.setCellValue("SVUG1000");

      HSSFRow row2 = sheet2.createRow((short)0);
      HSSFCell cell2 = row2.createCell((short)0);
      // The space in the sheet name causes the set formula to throw an 
exception
      cell2.setCellFormula("Test Sheet1!A1"); // Link to cell 0 on sheet 1

      book.write(out);
      out.close();

    }
    catch (Exception ex)
    {
      System.out.println("Ex: " + ex);
    }
  }
}