A smart guy fools a gambling site (which accepts unconfirmed deposits but allows withdraws only after all deposits are confirmed) by depositing bitcoin in low priority transactions (unconfirmed inputs, no fees). When he doubles his wage he waits a day until his low priority deposit is finaly confirmed and cashes out. When he loses his wage he somehow achieves that his deposit do not confirm at all and just creates a new account.
What I don't understand is why one tx gets confirmed and the other one does not. Does not look like a double spend to me but maybe I'm misinterpret blockchain data.
Here are two example transactions:
Confirmed tx (and I have no idea why this one went trough but the other did not):
https://blockchain.info/tx/3c2ac5ce691d99770c654f4fa2a69b1825e93158fedc3436de07f18bb6f91bf4
Unconfirmed tx (and I'm pretty sure it won't confirm):
https://blockchain.info/tx/1f006d66ea95ce46cafe4be66c149d7299fd35d4aa7287e5db4f435dce66eda1