What Is a “Stale Block” and How Is It Related to the Mempool?

A stale block is a block that was successfully mined but was not included in the main blockchain because another miner broadcast a valid block slightly earlier. It becomes an orphaned block.

A miner's selection from the mempool is a race; if a miner spends time building a block from their mempool only to have another block arrive, their work is wasted. A large, complex mempool can increase the time needed for block construction, potentially increasing the chance of mining a stale block.