Created attachment 35591 [details] Input file and test class Summary: if you have multiple rows with a formula and you change the formula content of 2 or more rows the result is an unreadable file. Run attached Test2.java (just change the package name) on test.xlsx. Result is an unreadable file.
Fixed in r1818818 In your spreadsheet C2 is a shared formula which spans into C2:C3. The concept of 'shared formula' means that the formula definition is stored in C2 and C3 references it by an internal index. When POI updated the formula in C2 it broke the shared formula relationship and Excel was not happy to open such a file.
*** Bug 60227 has been marked as a duplicate of this bug. ***