The audit you owe a complaint
When someone says your system is wrong, the first move should be to go look -- not to explain why it isn't.
I went back through the raw history because someone said the number was wrong. Message by message, timestamp by timestamp. I didn't want to. I had built the thing, I had tested it, and my first instinct was to explain how the number was actually correct, or how some edge case had created a one-time discrepancy that wasn't really a discrepancy.
Instead I looked, and she was right. My tool had taken two separate pieces of information from two separate sources, treated the second as a correction of the first, and quietly deleted the original. Not a crash. Not an error message. A confident, authoritative, wrong number sitting in a log that looked completely clean.
The record said ten minutes. She had done over two hours. I had a system with timestamps and a history view and everything it needed to look trustworthy, and it had lied.
Defense is the default
When someone disputes your output -- a number, a price, a record, a result -- the first draft of your response is almost always some version of explaining why the output is correct. Founders are especially prone to this. You built the thing. You understand how it works. The complaint reads as a misunderstanding of how it works, not as evidence that it doesn't.
The discipline is to go look before you open your mouth. Not to summarize the data. To find the original event, trace it forward, and understand exactly where the gap opened.
In this case it took thirty minutes. I found the exact source, matched it to the exact log entry, understood the exact failure, fixed the root cause rather than the symptom, and added two guards that make this class of error structurally impossible.
Andrew Hunt and David Thomas wrote that it's not important to get it right the first time -- only vitally important to get it right the last time. I used to read that as forgiveness. Now I read it as a requirement. Getting it right the last time means you actually went back. You actually checked. You didn't count the investigation as done when you had a plausible explanation.
The record was corrected. The apology was short. I still don't know why going to look is so much harder to make yourself do than explaining.