steps to recreate bug (all numbers are just an example) :
wb=openpyxl.Workbook() ws=wb.active ws.cell(2,3,"=RC") wb.save("temp.xlsx")
now open excel (2013, x64, win7) - it'll give this error:
clicking YES: loads the file without the formula (but with everything else).
clicking NO does not load the file at all.
RC is a special Excel construct. When RC is entered in a formula, it is immediately converted to an address.
It's probably not OpenPyXL's fault at all, since OpenPyXL doesn't officially supports R1C1.