What's the biggest mistake first-time founders make?

If you've been building startups for a while, you've probably seen the same patterns repeat.

Some founders spend months building before talking to customers.

Some raise money too early.

Some wait too long to launch because they want everything to be perfect.

Others focus on features instead of solving a real problem.

Looking back at your own journey (or the startups you've worked with), what do you think is the single biggest mistake first-time founders make?

More importantly, what lesson did you learn that changed the way you build?

Share your experience below. Your insight could save another founder months of wasted time.