When two miners find blocks at nearly the same moment, the network briefly holds two candidate histories. Whichever chain is extended first wins, and the other block is discarded along with its reward.
Stale rates are one reason real payouts fall slightly short of theoretical expectation, and one of several things a simple profitability model does not capture.
This definition is educational and not financial advice. Crypto is volatile and high-risk — always do your own research.
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