There are some things you can only learn in the hard way. It didn't happenned to me personally but to my team friends, so I consider it as it is my bad.

We upgraded our ETL tool - Informatica, from version 8.5 to 8.6. We had to run some tests to see that the results are the same. So, what we did is to save the result table from 8.5 in Excel, save the result table from 8.6 in Excel and then compare them using Excel-built-in functions. The only problem is we didn't pay attention to the places where zero and null interchanged. This happened because the two versions act differently where null values take places in aggregation functions. For example, when there is a sum function and it aggregates only on null values. In one version the output is zero and in the other the output is null...

This can also happen in other tools and technology. For example, in OLAP cubes the difference between zero and null is the difference between seeing the member of the dimension on the screen and not knowing of its existence.

For conclusion, always be aware to this point and don't forget to check it.