Continuing to Double Bet until you win...

[Pat Southern]

My friend was trying to tell me about a way gamblers can win no matter what they're betting on. Say you make a $100 wager, if you win you quit ($100 profit) or make another $100 wager. If you lose, then your next wager you make $200, if you win that then you start over at $100 ($100 profit). If you lose the $200 bet then you make a $400 bet, if you win you go back to $100 ($100 profit)....

Anybody have any thoughts on this for blackjack, sports betting, etc? Am I missing something other than the fact you'd need a huge bankroll to actually make this work?