Consistency between HSSFCell, XSSFCell, and SXSSFCell: HSSFCell throws an error on blank cells despite the javadocs saying 0 is returned. XSSFCell throws an error on blank cells. SXSSFCell returns 0 on blank cells. If we upgrade getErrorCellValue() from returning a byte to a FormulaError enum, then we would either need to add an enum value corresponding to "no error" or make the functions throw type mismatch (InvalidStateException) like the other cell types. I'm in favor of making SXSSFCell behave the same as HSSFCell and XSSFCell here and updating the documentation.
I think that fetching an error value from a non-error cell should behave like fetching a string value from a non-string cell. So, I'd vote for updating the javadocs and having the InvalidStateException in SXSSF