👋 Hello, this is Valentina with the free edition of the Optivem Journal. I help Engineering Leaders & Senior Software Developers apply TDD in Legacy Code.
Hitting 100% code coverage doesn’t mean your code works. Here’s why ATDD matters.
I remember when my client’s team first got obsessed with code coverage.
“Let’s hit 90% coverage this sprint!” the manager said.
Great, right? Except… two months later, they had 95% coverage and yet the system kept breaking in production.
I learned something hard that day: coverage is just a number.
It doesn’t tell you if your code works, if your business rules are correct, or if anything is maintainable.
You can execute every line of code and still ship disasters.
Knight Capital Group – $440 million loss in 45 minutes
Here’s one incident that still keeps me up at night.
In 2012, Knight Capital Group deployed an update to their trading system.
An old, previously disabled piece of logic got re‑activated by accident.
The system started sending orders it shouldn’t have.
Within 45 minutes, they lost $440 million. Just like that.
Here’s the kicker:
No amount of code coverage—90%, 95%, or 100%—would have prevented the disaster.
Why? Because this wasn’t a line‑not‑executed problem. Only Acceptance Tests could have caught this before it hit production.
Metrics can make you feel safe. Tests that focus on behavior actually keep you safe.
Code Coverage is a Trap
Code coverage only tells you code ran.
Not that it worked.
Running code ≠ verifying behavior.
I learned: chasing metrics doesn’t make you a better developer—thinking in terms of behavior does.
Metrics ≠ Safety
Next time someone asks about coverage, ask: “Does it actually tell me my system works?”
If the answer is no, stop worrying about the percentage and start thinking about behavior.
This is exactly why I rely on Acceptance Tests. Unit Tests with 100% code coverage are not enough.
They protect you from the stuff metrics can’t see—accidental behavior changes, re-activated logic, “how did this even happen?” bugs.
If you want to learn how to write acceptance tests that make production failures boring instead of catastrophic…
🚀 Join me for Acceptance Testing (Live Training) on Wed 25th Feb (17:00 - 19:00 CET).
P.S. Regular price: $97 (free access for Optivem Journal paid subscribers - see event description for details)
When you keep getting pressure to hit coverage numbers - how do you explain to managers that it’s not the same as quality?