What's one lesson that completely changed how you build products?

As I work on my next product, I've realized that many assumptions I had when I started building no longer hold up.

For example, I used to think shipping more features would make a product more valuable. Now I'm questioning whether solving one painful problem exceptionally well is usually the better approach.

I'm curious about the lessons that permanently changed how other founders build.

Some examples could be:

• A mistake that cost you weeks or months.

• A customer insight that changed your roadmap.

• A product decision you would never repeat.

• A piece of advice that turned out to be surprisingly true.

I'd love to hear the stories behind those lessons rather than just the lesson itself. I think those experiences are often more valuable than generic startup advice.