Marathon Mined an Invalid Bitcoin Block—Here's What That Means

Public Bitcoin mining firm Marathon Digital mined an invalid Bitcoin block on Wednesday, causing the firm to miss out on newly minted BTC it would have otherwise earned.

Various Bitcoin users and developers including Casa CTO Jameson Lopp identified the invalid block using their own Bitcoin nodes earlier today. Marathon later confirmed that they were in fact responsible.

An invalid block is one that violates Bitcoin’s consensus rules, and is therefore rejected by network nodes. Bitcoin nodes are run by miners, exchanges, and ordinary users alike, who keep their own copy of the blockchain and verify new blocks of transactions as they come in.

A block may be labeled invalid if it includes a double-spend transaction or breaks Bitcoin’s block size limit, among other things. Marathon did not specify how its block was invalid, though some onlookers attributed the problem to a transaction ordering issue.

In an email to Decrypt, a Marathon spokesperson confirmed that the company has never made this error before, calling it “an anomaly.”

“There were some people who were wondering if the invalid block may have stemmed from an issue with Bitcoin's protocol—it did not,” the spokesperson added. “Bitcoin functioned exactly as it was designed to.”

“The error was the result of an unanticipated bug that came from one of our experiments,” explained Marathon to X (Twitter) on Wednesday. The firm said it likes to experiment using a “small portion” of its hash rate to research how it can “optimize operations.”

“Our team noticed the invalid block around the same time as the rest of the world, and we immediately corrected the error,” added the firm.

Since the problematic block was noticed, shares for Marathon – which trades on Nasdaq under the MARA ticker – have fallen 2.9% on the day.

“It appears as if MARA had transactions in the wrong order,” commented BitMEX Research on the matter.

The block included a transaction (A) that was spent from an output of another transaction within the same block (B). “However, txin B was included in the block after txin A, therefore the block was invalid,” BitMEX continued.