Don't optimize against the test set
If you tune parameters and validate on the same window, your backtest is a souvenir, not a forecast.
Most strategies that look brilliant on a single backtest die in the first week of live trading. Curve-fitting to a specific window is the single most common reason — the parameters discovered describe the past, not a tradable edge.
A simple split
Tune your parameters on the older 60% of available history. Hold out the most recent 30% to validate without touching it. Leave the last 10% as a forward-walk check after you deploy.
Red flag