If your validation set isn’t truly out-of-sample, you’re just fooling yourself. Time leaks, target leaks, and accidental peeks are everywhere. Validation is only as honest as your split logic. Always simulate real-world conditions: shuffle with care, respect time order, and never let future data leak into the past. Double-check your splits and challenge your assumptions... your model’s credibility depends on it. [[Data Leak]] [[ML]] [[Serendipity]]