← Adam Garceau

Claude Found the Excel Mistake That Overbilled a Customer

October 16, 2026 · The written version of my video. Reading time: about 4 minutes.

A formula reading the wrong row had overbilled a customer $46.92, and nobody caught it. I had AI check the math inside a spreadsheet at a small business I operate in, and that's what it found.

When's the last time anybody checked the math in yours? The same check works on any invoice, price list, or inventory sheet, and you can run it on a copy with the customer's name taken out.

Why I'm allowed to show you this

I'm not an outside consultant selling something I did for a stranger. It's a family business that has been running for about 20 years. A few weeks ago I took over the books and operations work, which is why I know these numbers are real. This isn't a case study I bought. It's a spreadsheet I actually had to open.

The process before I touched it

Every week someone rebuilds an invoice by hand:

• They open last week's spreadsheet, save it under a new name that never really gets updated, and start typing.
• A paper invoice arrives from a vendor. Someone reads the case counts off the paper and keys them into the sheet.
• That number gets typed a second time into a different system to submit it.
• Payment reports arrive as a PDF. Someone retypes them into a spreadsheet tab, matching machine numbers from memory.

None of that is stupid. It's what a business looks like after 20 years of running the same way with no reason to change it.

Here's a question to sit with: how many times a week does somebody in your business type the same number twice into two different places by hand? If you don't know, that's not a knock on you. Almost nobody has ever had a reason to count it.

What the AI found

I went through the actual spreadsheet, formula by formula. Two problems:

1. A formula reading the wrong row. It had quietly overbilled a customer $46.92. That's the point: $47 is small enough that nobody notices, which is exactly the size of mistake that survives for months.
2. A price baked into a formula that didn't match the price next to it. Same product, two different numbers on the same sheet, in two places a human would look right past.

Neither is a story about a careless person. It's a spreadsheet nobody had gone through in a long time, because everyone was too busy running the business to audit the thing that runs the business.

How I did it (no code)

I'm not a coder or a developer, and I didn't write a script to find the errors. I described the sheet to an AI tool in plain English, told it what the invoice was supposed to add up to, and asked it to walk through every formula and tell me where the math didn't hold up. Then I verified what it found against the actual paper.

That's the whole method:

1. Make a copy of the sheet, and take out customer names.
2. Upload it to Claude. The free plan works.
3. Say what the sheet is supposed to produce.
4. Ask it to check every formula and flag anything that doesn't hold up.
5. Check each flag against the paper before changing anything.

The finding that scared me more than the money

While going through old records, I found a full card number sitting in plain text in an old email. A wrong formula costs you money slowly. A card number in an inbox costs you all at once, the day somebody who shouldn't see it does. If you've been in business more than a few years, search your own email for "card number" and "password" right now.

Step two: so they don't come back

Finding the errors was step one. Step two was building something so they don't return.

• A form on a phone so whoever's out on the route enters the count once, at the stop, instead of writing it on paper and retyping later from memory. That one is built and in use.
• A checker for the weekly invoice. The rule is simple: every line has to equal price times quantity. If it doesn't, it stops and flags it instead of quietly going through. Some of these are built, some are on the list.

Neither took a computer science degree. They took someone willing to sit with the actual mess and ask an AI tool to write questions about it.

About the hours saved

It would be easy to throw out a big "hours saved" number. I'm not going to. Nobody ever put a stopwatch on the old process. The only time figure on record is a rough personal estimate of the whole Friday block, and I won't dress that up as measured data. I can tell you what was wrong and what's fixed. I can't tell you yet how many hours a week it's worth, and I'd rather say I don't know than make up a number that sounds good on camera..

Your turn

If any part of your business still lives in a spreadsheet that gets copied and renamed every week, there's almost certainly a mistake sitting in a formula somewhere. You don't need to learn code to find it. You need to ask in plain English, "check this math for me," and then look at what comes back.

Everything here runs on your own accounts with your own files, and you keep it. If this is you, comment and I'll walk you through what to look for..

Next: the exact math on a software bill. How a Small Business Owner Decides: Replace This Software With AI or Keep It.

I'm a marketer who builds his own tools. More at adamgarceau.com.