Setting the font of a cell using CellUtil.setFont(cell, font) always creates a new style in the workbook, regardless of whether the font is new or not, and may therefore lead to "IllegalStateException: The maximum number of cell styles was exceeded". 
Problem seems to be that the private putAll method in CellUtil stores a Short value for the font in the map, whereas getFormatProperties(CellStyle style) uses an Int, so that the "if (wbStyleMap.equals(values))" call in setCellStyleProperties will always return false. 

Steps to reproduce:

        HSSFWorkbook wb = new HSSFWorkbook();
        HSSFCell c = wb.createSheet().createRow(1).createCell(1);
        HSSFFont f = wb.createFont();

        CellUtil.setFont(c, f);
        int num1 = wb.getNumCellStyles();

        CellUtil.setFont(c, f);
        int num2 = wb.getNumCellStyles();

        System.out.println(num1 == num2); // should print true, prints false