What’s the most expensive Google Ads mistake you’ve made?

I’ve managed Google Ads for small businesses for 12+ years, and the costliest mistakes I see usually aren’t the ads. They’re decent ads sending clicks to pages that don’t deliver on what the ad promised. I’m launching CouncilAds on November 8 to catch that before the money is spent. Curious what’s burned you: bad targeting, weak copy, the landing page, or something else?