You are doing an audit to see if what you installed actually works the way you expected it to! There are so many little bugs that can occur, and there will always be issues. The object is to catch them immediately while you have the opportunity to easily correct them, or modify the new program to what you want it to do.
An example - We had a switch, where a user defined field on the new program meant "drop a statement every day" instead of outsort for commercial loan review. (Our mailroom was happy that was corrected.)
It's time consuming, but worth it.
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Integrity. With it, nothing else matters. Without it, nothing else matters.