A good example is betting on roulette.
If you keep doubling your bet, you will always win.
So bet €1 on black, and black comes up, you win €1.
Bet it again, and if red comes up, you lose €1.
So the next time bet €2 and if black comes up, you win €2, so your total win from this second series is €1.
Then you run into 4 reds in a row but black comes up on the 5th roll.
1) Bet €1 - lose €1
2) Bet €2 - total losses €3
3) Bet €4 - total losses €7
4) Bet €8 - total losses €15
5) Bet €16 , black comes up - cumulative win €1.
So your "system" is working.
But eventually you will hit a run where you lose everything.
Brendan