While celebrated for their low costs and broad diversification, index funds, designed primarily to match index returns, face hidden costs related to their need to trade to track market changes. These costs stem from adverse selection (buying when firms are selling and vice versa), price impact (buying after prices rise due to index inclusion and selling after they fall due to exclusion), and mean reversion (buying high-valuation stocks that tend to drop and selling low-valuation ones that tend to rise). These trading behaviors, while necessary for rigid index tracking, deviate from maximizing investment returns, suggesting that a more flexible approach could improve performance despite the valuable nature of index funds as an investment vehicle.