The 6 most dangerous A/B testing mistakes (and how to avoid them)
After 15 years of shipping products, I’ve seen A/B tests go wrong in every way possible.
Here are the 6 most dangerous mistakes product teams make — and how to avoid them:
1. Peeking Too Early
This one kills me every time.
You plan to run the test for 4 weeks.
But 3 days in, the numbers “look promising,” and someone says:
“We should just ship it.”
That’s not experimentation. That’s guessing in a lab coat.
If you want reliable results:
- Stick to your sample size
- Or use sequential testing the right way
2. Chasing Too Many Metrics
If you look at 20 metrics, one is bound to win.
That’s not insight — it’s statistical roulette.
Do this instead:
- Pick one primary metric
- Use guardrail metrics to ensure nothing breaks
Clarity beats complexity. Every time.
3. Forgetting Segment Impact
I’ve seen tests that looked amazing overall…
But quietly tanked new user onboarding.
If you only check the top-line number, you’re playing with fire.
Always slice the data:
- New vs. returning
- Mobile vs. desktop
- Free vs. paid
The truth hides in the segments. Go find it.
4. No Clear Success Criteria
This one’s a silent killer.
“Well… it wasn’t worse, so maybe we should ship it?”
That’s how teams ship neutral or even harmful features.
Set expectations before you run the test:
- What metric needs to move?
- By how much?
- Over what timeframe?
No post-hoc rationalizing. Period.
5. Obsessing Over One Metric
Optimizing conversion? Awesome.
But if it kills retention, speed, or satisfaction…
Did you really win?
Build a metric system:
- Primary = what you want to move
- Guardrails = what you can’t afford to break
Holistic wins > tunnel vision.
6. Overfearing Experiment Interaction
Many teams slow down testing, scared one test might “interfere” with another.
Unless you’re testing conflicting flows in the same UI,
interaction effects are rare.
I’ve run thousands of tests.
Parallel testing has never been the bottleneck.
Don’t paralyze progress. Just prioritize smartly.
A/B Testing Isn’t About Volume. It’s About Learning Velocity.
How fast can your team generate trusted insights?
That’s your real edge.
Run smarter, not just more.
And remember: Every test is a chance to get better at building.
