I never know there's a chance of cancelling a transaction which is already sent on the blockchain and I go through this thread but still don't understand how the process was done since OP don't explain in length and ETFbitcoin message too tech for me to understand very well.
He sent another transaction which spent the same funds, but to a different adress with much, much higher fees.
So the miners added his transaction instead of the other one, due to the bigger monetary reward it would get them.
The transaction was never ‘canceled’ it was simply overridden by another one because the “first” transaction wasn’t confirmed yet.
If it was already confirmed (a miner has already mined a block including his transaction), it wouldn’t have been possible to do what he did. (Unless someone does a much more sophisticated attack, which usually isn’t worth it or even possible in the first place.)
Basically:
https://coinsutra.com/bitcoin-double-spending/Thank you, that's an excellent explanation that even a noob like me can digest.
