This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
... how fast transactions propagate around the network, it’s tricky to get the timing correct, but assuming you do, the merchant will think he received a valid payment, while the miners will incl ude double spend in their blocks. This can be done with a very high success rate and is probably responsible for almost all of the ‘zero confirmations transactions are trivially reversed’ me me.

Well not so fast

While we can indeed double spend unconfirmed transactions with a very high success rate, the merchant should be able to trivially detect mos...
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