When you get down a number of cells, you start noticing errors, as I have shown in both row 51 and 54 in this screen dump next to this text. 20 multiplied by 2 should be 40 - not 50. 500 multiplied by 2 doesn't give you 990 etc...
I fully realize this won't affect your personal budget, but this type of error is rather interesting, because we did this experiment almost twenty years ago with exactly the same result, and both OpenOffice and Microsoft have this error in their spreadsheet software.
It might not be useful for much, but you can at least refer to this next time someone wants to make the claim that software is error-free...