The latest incident occurred at block height 966,500. Two different mining pools produced valid competing blocks built on the same parent block.
SpiderPool's version was treated as the tip of the Bitcoin blockchain after reaching Galaxy Research's node. However, AntPool had independently produced another valid block at the same height. The aforementioned block ended up becoming part of the canonical chain.
It is worth noting that there was nothing wrong with the Bitcoin network. There was no consensus bug or successful attack. Short-lived competing blocks are completely normal despite the fact that the word "reorganization" might seem scary.
What actually happened?
According to Galaxy, AntPool's ultimately successful block 966,500 had a timestamp of 11:30:30 UTC and contained 4,359 transactions. SpiderPool also produced a valid block 966,500.
Galaxy's node saw that block first. Hence, from the perspective of that particular node, SpiderPool temporarily represented the head of the blockchain.
This is possible because Bitcoin blocks do not reach every computer on the network simultaneously. A miner in one part of the network might therefore see SpiderPool's block first. Meanwhile, another miner might already be working from AntPool's competing block.
Notably, Galaxy's node was initially unaware that there were two versions of block 966,500.
That changed when AntPool produced block 966,501 on top of its version.
Bitcoin nodes follow the valid chain with the greatest accumulated proof-of-work. Galaxy's node therefore abandoned the SpiderPool version and switched to AntPool's branch.
The SpiderPool block became what is commonly called a stale or orphaned block.
SpiderPool's discarded block contained a reward of roughly 3.1389 BTC, according to Galaxy's graphic. The reward subsequently became unusable.
AntPool's winning block, by comparison, earned about 3.1411 BTC.
Third case in less than a month
Galaxy Research says the previous two one-block reorganizations occurred at heights 962,722 on Aug. 16 and 963,853 on Aug. 24.
The Aug. 16 case involved competing blocks at height 962,722. Another reorg followed on Aug. 24 at block 963,853.
As mentioned above, a one-block reorg of this type is generally not considered concerning.
