Plans that were doomed from the start: what gave them away?

Welcome to the home of ideas that had one obvious flaw everyone could see except the person carrying them out. Jumping onto the frozen pond, lighting the barbecue with a bit too much fuel, reversing the trailer while filming it. We watch these for the laughs, but they are secretly a crash course in common sense. So let us talk: What is the clearest warning sign that a plan is about to fail? Which classic mistakes keep repeating across the world? And which clips taught you a lesson you actually use? A small house rule: laugh at the idea, not at anyone who got seriously hurt, and skip anything graphic. Post your favourite examples, add your prediction in the comments before revealing the outcome, or trade near-misses in the chat.