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.
@kekcoin Thing is, since we don't know what that transaction was for, Andreas can _not_ claim it wasn't a double spend, even by a definition that requires someone to get ripped off. We just don't know if someone did or did not lose money. I'm _guessing_ no. But I can't be sure.
@pete That's a good point.
@pete That's a reasonable and internally consistent definition. No guarantee Andreas is using the same one, though