Andreas is incorrect here. A double spend _did_ happen.

Bitcoin's double-spend protection is probabilistic: after one confirmation, if the sender is attempting to double spend, the probability of success is extremely low. But it's still non-zero.

As the Bitcoin whitepaper helpfully explains, the probability of a double spend attempt succeeding drops exponentially with the number of confirmations. One confirmation is extremely low, two is basically (extremely low)².

Full details: https://bitcoin.org/bitcoin.pdf

Unfortunately, Andreas falsely claiming a double spend did _not_ happen is dangerous: people have to realize that a single confirmation is not an absolute guarantee.

This case was ~$20, and looks like someone was just moving money between different wallets.

But if you're accepting a payment large enough that you can't risk even a very low chance of a double spend, you _do_ need to wait for multiple confirmations. Claiming otherwise could lead to people losing money.

@pete That depends on how you define "happened". Is the fact two conflicting txes exist, unmined, in "the" mempool enough for a double spend to happen? Is the fact that two conflicting txes were both mined in competing blocks enough? Why is one more "happened" than the other?

@kekcoin I define a double-spend as happening if wallet software would have likely observed one happening.

So double-spends of unconfirmed txs happen *constantly*. I personally have services that do them dozens of times every day.

Double-spends of txs with 1 confirmation are very rare. But they still occasionally happen.