This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...s will mine the next block containing your double spend. A 3% chance of success is still fairly low, but you can repeat this over and over again until it works.

As damning as this s
eems to zero confirmation transactions, this too is detectable. The sys admin in charge of the retailer’s POS software could just add his own “is notorious” check to incoming transactions and push the “payment declined” message to the register if such a transaction is detected. Problem solved.

Of course if you have to do this for every mining pool, it can be cumb...
read full book block explorer