Created attachment 35448 [details] example to create a valid (XSSFWorkbook) and a invalid (SXSSFWorkbook) workbook If a workbook of type SXSSFWorkbook is created with a array formula, the workbook will contain invalid data.
It fails because currently SXSSFSheet tries to use XSSFSheet for storing the array formulas, but this causes rows to be created in the sheet at SXSSFSheet._sh and this way some rows are written twice to the sheetX.xml file. Full support for array formulas is likely some more work as they will need to be kept in the SXSSFSheet and "merged in" whenever normal rows are written to the xml-file instead of being added to the XSSFSheet.
With r1819770 we throw an "Unimplemented" Exception when you try to set an array formula for SXSSF to make this more visible instead of creating corrupt files.