What Is a Stale Block?
A stale block refers to a block that has been successfully mined but is not included in the current longest blockchain.
This typically occurs when another block at the same height is added to the chain first.
Stale blocks can result from network latency and temporarily lead to a split in the network, creating two competing blockchains.
Miners resolve this split by continuing to mine new blocks on the valid blockchain, eventually causing other miners to follow suit.
The stale block, part of the chain no longer actively mined, is considered invalid.
The mining reward associated with the stale block is also invalidated and cannot be spent.
The transactions within the stale block return to the mempool and are subsequently included in future blocks.
What Causes Stale Blocks?
The occurrence of stale blocks is relatively low in Bitcoin due to the low latency between mining pools.
When miners successfully mines a block, they broadcast it to the nodes closest to them, propagating it to nearby nodes, and so on.
However, data transmission is not instantaneous, allowing another miner to find the solution to the same block simultaneously.
Stale Blocks on Ethereum and Other Blockchains
Before Ethereum transitioned to proof-of-stake, stale blocks, referred to as “uncle blocks,” were possible on the Ethereum network.
Miners could still earn a reduced reward from these uncle blocks.
Stale blocks can also occur on other proof-of-work blockchains, although they are more common than Bitcoin.