The most common mistake beginners make in blackjack is not using basic strategy, which is a set of mathematically proven decisions for every possible hand. This often leads to players making instinctive or emotional choices, such as hitting when they should stand, or standing when they should hit, resulting in higher losses. Beginners may also mismanage their bankroll by betting too aggressively or inconsistently, which increases the risk of losing large sums quickly. Additionally, many new players fall for the myth that certain hands or card patterns guarantee success, when in reality, blackjack is a game of probabilities, and the right strategy is key to minimizing the house edge.