It is very possible to lose big in online Blackjack. However, it usually takes time to do it. Blackjack is not like Poker, where the pot grows with every hand and you can win or lose based on how much you throw in — in Poker, you can suddenly decide to bet everything in the middle of a hand and lose big. In Blackjack, you put your money down at the beginning. You then play out the whole hand without increasing or decreasing the amount that you want to bet on the hand. It stays as it is. The hands go by fairly quickly, so you will soon be on to the next hand and will be betting again.
Because of this, the way to lose big is to lose a lot of times in a row. Sometimes you have streaks where luck seems to go against you, where the House keeps on hitting twenty-one on the head or you keep busting when you Hit on low numbers like twelve. This could be because of luck alone, or it could be because you have a bad strategy. In either case, you could end up losing a lot of hands in a row and watching your bankroll deteriorate in a hurry. This is a sinking feeling, rather like sliding on the ice in your car. You desperately want to stop and you just cannot seem to do it.
The best way to guard against this is to bet the same amount every hand. This way, you will gain as much as you lose. When your luck turns, you will get all of that money back as quickly as you lost it. This can make it very easy to dig yourself out of a hole and find a lot of success. This also keeps you from losing all of your large bets and winning only your small bets. If you do that, you could end up losing money overall even if you are winning the majority of the bets that you make. If you can afford it, bet the table max on every hand to have consistent play.