6 Testing Mistakes You Should Avoid

Each with a simple fix you can apply today

50% OFF - The Complete TDD course

Are you ready to master Clean Code, Testing and Test-Driven Development (TDD)?

I recently launched a complete TDD course containing everything you need to craft high-quality software.

Now there is a 50% OFF for the course

Get instant access by clicking here.

Motivation

Most developers are bad at testing. Not because testing is hard, but because they keep making the same mistakes over and over again.

I’ve helped many teams and developers, and I’ve seen the same 6 mistakes pop up every single time.

The good news? If you avoid these, I guarantee you will be better than 90% of developers out there. Let’s get started!

❌ Mistake #1: Testing Too Late

✅ Do shift-left testing instead

Tests aren’t just for verification. They’re powerful tools for learning, understanding, and designing software. This is what shift-left testing is all about: moving testing earlier in the development process.

The earlier you test, the easier, faster, and cheaper it becomes to fix issues.
It leads to higher quality, faster feedback, and more confidence.

Remember: the earlier you test, the better everything gets.

❌ Mistake #2: Focusing on Code Coverage

✅ Use Mutation Testing instead

Code coverage is NOT a quality metric. You can have 100% coverage with terrible tests and zero assertions. Coverage helps detect untested areas of the code, but it doesn't tell anything about the tested areas. It gives you a false sense of security.

How can we measure the quality of our tests then?

The answer is: Mutation Testing. It is a technique that makes small changes to your code and then runs your tests. It is expected that at least one of your tests will fail. If not, it might indicate that your tests do not fully cover your code.

Code coverage won’t tell you how good your tests are. Mutation Testing will.

❌ Mistake #3: Testing Implementation Details

✅ Test behaviors via public APIs

I get this question a lot: How do you write tests for private functions?

Answer: You don't

They are implementation details. If you couple your tests to implementation details then they become fragile and break during refactoring. If your tests often break, they lose their value.

Don't couple your tests to code structure.

Couple them to the behaviors of the public APIs

❌ Mistake #4: Focusing Only on Clean Code

✅ Focus on Clean Tests as well

Most developers obsess over clean production code: naming, structure and SOLID principles.

But then they treat tests like second-class citizens:

• Cryptic method names

• Copy-pasted setups

• Insufficient assertions

The result? Tests become fragile, hard to read, and impossible to trust.

Clean code dies fast without clean tests. Your test suite should be your safety net, not a trap. Clean tests are simple, readable, and tell a story. They give you confidence to refactor, move fast, and stay in control.

If your tests feel like a burden, it's time to clean them up.

❌ Mistake #5: Doing Test-last

✅ Do test-first with TDD

Test-first beats test-last development every time:

• Test-first is fun - test-last feels like a chore

• Test-first helps you design code - test-last only verifies code

• Test-first puts your API under pressure - test-last just accepts it

• Test-first shows if a test is meaningful - test-last can lead to false positives

• Test-first helps ensure uniqueness - test-last often causes duplication

Test-Driven Development (TDD) is the ultimate form of test-first. It lets you design your code by writing automated tests that shape your software.

❌ Mistake #6: Testing Only Within Your Team

✅ Test together with your users

Developers tend to forget the most important testing type. It is not security, integration or performance testing.

It's beta testing:

• It's where your software meets reality

• It's where your customers interact with your product

• It's where you get real-time feedback

It's not about finding bugs. It's about learning the domain and shaping the software with the customer together.

Clean code helps you build the software right.

Beta testing helps you build the right software.

Do it every week.

Conclusion

The best way to build confidence in your code is to test it well. If you want to master testing, Test-Driven Development and Mutation Testing check out my recently launched complete TDD course, which includes:

• The fundamentals of Test-Driven Development

• Three real-world TDD examples in C#, TypeScript and Rust

• The power of Mutation Testing

• Using TDD to design high-quality software

• Testing legacy code

• Refactoring best practices

Get instant access by clicking here.