The most expensive funnel mistakes don't break anything. They just make the numbers slightly worse, every single day, until you've trained yourself to accept a 1% conversion rate as normal.
One: too many choices. Every extra button, link, and navigation item is an exit. A focused page with a single action almost always outperforms a busy one.
Two: asking for too much, too soon. If your first form has nine fields, you're filtering out people who would have happily given you their email. Earn the rest of the data later.
Three: no clear next step after conversion. Four: slow load times on mobile. Five: a CTA that describes the action ('Submit') instead of the outcome ('Get my free plan'). Fix these five and most funnels see double-digit gains.