An arbitrage exists when discrepancies between bookmakers let you back every outcome and lock in profit regardless of the result — the combined implied probability of your chosen prices falls below 100%. Arbs appear when books disagree or lag on news, and typically pay 1–4% of total stakes.
The catches are practical. Arbs demand large turnover for small returns, precise stake calculations, and speed — prices vanish in minutes. Bookmakers detect the pattern quickly and respond with stake limits or account closures. Voided bets at one book can leave the other leg dangerously exposed. It is a real but grinding trade, closer to logistics than to betting.
Book A: home 2.10 (47.6%). Book B: away+draw (X2) 2.15 (46.5%). Total 94.1% < 100%. Staking £51.20 and £48.80 respectively returns ~£107.50 whichever way the match goes — about +5.9% on £100 total.