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It's not easy to beat a casino consistently. Many gamblers get overconfident after winning once or twice and end up making bad decisions. If a gambler really thinks that his strategy is effective, he will eventually be proven wrong, because it is difficult to beat the casino in the long run, and that is historically true. So a gambler must keep this in mind before becoming overconfident and making any wrong moves.
Winning big enough to beat the casino is certainly possible, but it's unlikely to happen at online casinos. Someone with specialized skills in gambling and certain games may have done it, but they're likely only successful at physical casino, not online one. Online casinos limit gamblers' winnings, usually stated in their terms and conditions. Each gambler can only win a specified maximum amount, which helps casinos avoid players winning more than they can afford to pay out.
Physical casinos are certainly different, we can actually find gamblers who have managed to achieve big wins that they didn't get at online casinos. So it makes no sense to try to beat the casino, especially when they are gamblers who gamble online.