Today there was a “stale block” on the Bitcoin blockchain. This is a block that is not added to the main chain because it has the same height as a previously mined block.
It is block 619970, of which two versions have been mined. Both blocks were mined at 10:32 AM (CET) on March 3rd, 2020, 4 seconds apart from each other, but only one of them had a subsequent block and entered the main chain.
The one that was mined first, which was then effectively integrated into the blockchain, was mined by OKEx and was linked to the next block, the 619971.
The other one, which was mined by Poolin, was discarded and remained without a subsequent block.
The two blocks do differ in the number of transactions included, the total volume of BTC contained in them, and the reward in fees.
There was no double-spending, and the 10 transactions included in the discarded stale block, and not in the other block, were then included in the next block.
This was revealed by BitMEX Research, which also published a diagram illustrating the two mined blocks almost at the same time, with the same height, but only one of them had a sequel and was therefore included in the main chain.
Bitcoin had a stale block this morning at height 619,970:
* 4 second time gap
* First stale block since 2020-02-27
* No double spends – The stale block contained 10 txns excluded from the competing block, which all made it into the next block (619,971)https://t.co/uzJXPLZduO pic.twitter.com/7Lr7tJFBto
— BitMEX Research (@BitMEXResearch) March 3, 2020
These are uncommon occurrences, but not rare, considering that the previous stale block had been mined on February 27th.
These are the result of conflicts in the Bitcoin network caused by the finite speed at which the various miners communicate with the nodes.
In this specific case, it took only 4 seconds of misalignment for Poolin to mine a block already mined by OKEx, which was then discarded by the network because it arrived later.
In reality, the reason why the first block, mined by OKEx, was included in the main chain of the Bitcoin blockchain, and the other one wasn’t, is that the first one had a subsequent concatenated block. In fact, in cases of conflict like these, the longest chain wins, so it’s enough that one of the two blocks has a following, and the other one hasn’t, resulting in the one without a subsequent block being discarded, even if it has been properly mined.