This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...versed (or ‘double spent’) by the sender, assuming he has some technical ability. This means someone could pay you for some merchandise, then steal back the coins afterwards. The more time t hat passes, however, the more difficult it becomes to reverse a transaction. Typically we say that “unconfirmed” transactions are the easiest to reverse, while the deeper a transaction is in the blockchain, the more difficult it is to reverse. After a transaction is buried six or so blocks deep in the blockchain (about an hour), the probability of a successful double spend drops cl...
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