Be careful when entering digits in Excel more than 15 digits long. Unless a field is formatted for text, Excel will replace the digits past the 15th one you enter with zeroes. See confirmation from Microsoft on this point here.
This is the kind of thing which is very easy to miss if you're working with lots of data.
See this example, where I type in a 19 digits number . . .
. . . but Excel automatically truncates the last four digits.