Test with your messiest real data first

I used to test new tools with clean, made-up examples. Now I test with my messiest real file first, the one with blank rows, weird spacing, and duplicate entries. That's the file that actually tells me if the tool works.

Clean test data flatters a tool and hides the exact problems that show up once it meets your real business. A customer list will have missing phone numbers. A form will have someone who typed their name in the email box. If your tool only survives a perfect example, it isn't ready, no matter how confident the coding session felt while building it.

Try this today: take the ugliest real file you have on hand, the one with gaps and typos, and run your new tool against that instead of a fresh example, then note every place it stumbles.