As far we know, some payment processors and users accepts 0/unconfirmed transactions. But this solution is not secure, because it’s possible to create two transactions with the same inputs, but different outputs.

http://eprint.iacr.org/2012/248.pdf

Attacker is able to send a first transaction directly to victim, and then broadcast a second transaction over the network. This couldn’t be prevented using the proof-of-work design, but it’s possible to fight with such manipulations through using an extension for our proof-of-stake system, and we even don’t need a chain fork to implement this.

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