The Martingale system works every time -- as long as you have an infinite amount of money. It tells you that if you lose, just double your bet until you win it back. You'll eventually win. The problem is, the math becomes so high so fast, there's no way you can cover your losses, expecially since casino limits will hit you.
Say you start off betting $5. You lose. Bet $10 to win it back. Lose. Bet $20. Lose. Bet $40. Lose. Bet $80. Lose. Bet $160. Lose. Bet $320. Lose. Bet $640 -- ooops, except you can't because the table limit is probably $500.
But even if it wasn't, you bet $640 and lose. Now you have to bet $1,280 on your ninth bet. What happens if you lose four more times in a row? You're betting thousands, all just to win back that initial $5 you lost.