Leakage is subtle and relentless. Every join, every timestamp, every “just this once” shortcut... assume it’s leaking until proven otherwise. Data leakage can invalidate months of work. Audit every step: are you using future information? Are your features truly independent of your targets? Build checks for leakage into your workflow, and treat every new data source as a potential risk. [[Data Leak]] [[ML]] [[Serendipity]]