BitMEX Research has identified a suspected double-spend transaction valued at 0.00062063 BTC or roughly $21 — and it doesn’t appear to be an instance of that popular replace-by-fee wallet hack.

On Jan. 20, BitMEX’s ForkMonitor noted that “multiple blocks were produced at height 666833.” BitMEX Research tweeted:

One hour later, BitMEX Research attributed the orphaned block to an RBF transaction, which is where an unconfirmed transaction is replaced with a new transfer paying a higher fee. However, ForkMonitor has since updated its advice to say: “No (RBF) bumps have been detected.”

Twitter-user and BSV’s Australian advocate Eli Afram noted the “mixed messages” from BitMEX Research, asserting the double-spent transaction should be cause for concern despite its small value:

“So it appears an actual Double-Spend has occurred on BTC... Not an RBF (replace-by-fee), but an actual double spend. A mere 22USD... but - this could have been 22million.”

Satoshi Nakamoto’s Bitcoin Whitepaper is credited with having solved the double-spend problem in 2009. The challenge of ensuring that a decentralized network can autonomously verify that the same coins have not been transferred more than once had stymied earlier attempts at digital cash. 

In July, crypto security firm ZenGo identified a double-spend exploit targeting several popular Bitcoin wallets. While the wallet manufacturers moved to address the exploit, Bitcoin Cash proponent Hayden Otto warned the vulnerability may be inherent to BTC’s replace-by-fee functionality. He’d earlier exploited the same vulnerability in a viral video.

Update: The headline to this article has been updated to note that BitMEX Research suspected a double spend. Subsequent analysis has determined that their suspicions were incorrect.