Create new XSSFSheet and some merged regions. Remove all merged regions and save workbook. When you try to open workbook with MS Excell it will propose you repair you excel file. Excell does not allows empty section "mergeCells" for sheet(see sheet2.xml in attached example). Solution: Use unsetMergeCells() of CTWorksheet after removing the last merged region on the sheet.
Created attachment 26002 [details] Example of corrupted file.
Created attachment 26003 [details] Patch.
Applied in r996148 Yegor