I worked for a company once where I was to implement some process' for the Month End.
So I reviewed the existing code.
Basically, it was written in Visual Basic 6.
There was an outer query, which looped thousands of times.
Each time it called an inner query.
I reviewed the code.
And found a bug.
So I brought it to my boss' attention.
He said "that is impossible, the entire company bases it's financial life on that query. The query IS correct".
I said "okay boss".
And that was that.
A few days later he called me into his office.
"So tell me again what is wrong with the query."
I showed him.
Sure enough it was wrong.
So he told me to fix it.
And then I reviewed the code for performance.
I removed the logic from Visual Basic Inner / Outer queries and put the code into SQL-Server stored procedure.
It reduced the run time from 5 hours to 10 minutes.
And then I was the month end specialist for the next 3 years.