Almost every strategy that dies in production looked great in a backtest. The backtest wasn't unlucky — it was wrong, in one of three specific, detectable ways. Here's each one, the exact test that catches it, and why your usual metrics never warn you.
1. Lookahead bias — the silent killer
It's almost never a deliberate shift(-1). It hides in subtle places:
Structural indicators computed over the whole series — swing highs/lows, pivots, "the trend", regime labels. If the value at bar t depends on bars after t, every signal derived from it is contaminated.
Global-statistic normalization — z-scoring with the full-sample mean/std, fitting a scaler on all data.
