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 Four confirmations has been the suggested minimum for several years. Do you agree with this? Thanks for your advice.
@theitaliandude Honestly, it depends on risk vs reward. As very rough guess, if someone is *actively* trying to attack you with a double spend, a reasonable worst-case guess is there's a 0.1% chance of them succeeding after one confirmation. And even less after two. But there's tail risk - that model doesn't capture it all.
@pete Makes sense. Thanks!