What's the most expensive PM decision you've made on bad data?

I've spent 30 years in product. Built and launched close to 115 products for companies like Sony Music and Zee Entertainment where I worked, for my own company and for companies that I consulted.

The most common pattern I've seen kill good products isn't bad strategy. It's decisions made with false confidence where the data looked complete but wasn't, and nobody said so out loud.

A few I've witnessed:

The pivot that wasn't. A team pivoted their entire ICP based on 6 customer interviews. All came from some or another referral source having similar profiles. They called it "validated discovery." NRR dropped 40% in two quarters.

The launch that couldn't be undone. A pricing change went out to all segments simultaneously because the readiness score "felt like an 8." They had no staged rollout and they didn't provision for the rollback. Three enterprise accounts churned within a few week.

The North Star that divided the team. Two product teams were optimizing against each other for an entire quarter before anyone noticed the OKRs conflicted. The conflict was visible in the data, but nobody looked at it.

 All of the above observations ended with the common thread: nobody graded the evidence. Every decision was presented with the same confidence regardless of how much data actually backed it.

This is why I built evidence quality grading into Lumen in which every recommendation is rated HIGH / MEDIUM / LOW, and the report explicitly calls out what data is missing.

But I'm curious about learning from your experience:

What's the most expensive PM call you've made or or seen being made on data that turned out to be thinner than it looked?

Discovery? Pricing? Launch timing? A pivot?

Drop it below. I am curious to learn and I'll share what pattern I see across the answers.