Market orders guarantee execution but not the exact fill price. Buying a large quantity of a thinly-traded stock at market can exhaust the shallow sell-side liquidity level by level, pushing the average fill price well above the quoted price — this gap is called slippage. For example, buying 500 shares at market when only 100 shares are available at $9.995 and the rest fill at $10.01, $10.02, and so on results in an average fill around $10.007.