Auditing if something goes wrong

While doing your reports, a figure may appear that looks wrong, and you need to investigate. 

This happened to me, a while ago while investigating our GST on purchases.

The way, I recommend you approach it is to isolate the problem to as small an amount and time as possible.

So what I did was break the period of the purchases into months and investigate each month, then in a month that was wrong investigated each week and then I went day by day till I found the error in the day.

Then I looked through all the transactions for that day till I found it, an insurance claim that had no GST.  

This method always in my experience works.

Note the toughest errors to find in my experience are those that look right.