Ran a backtest that looked incredible. Then realized it was leaking future data. The culprit? A feature calculated over rolling returns _including_ the current bar. Just a subtle off-by-one, but enough to poison the signal. Now every feature function explicitly enforces causality, no peeking past the target bar. I also added unit tests that assert zero leakage using synthetic data. Never trust a green equity curve unless you’ve proven it blind. [[ML]] [[Data Leak]] [[Serendipity]]