Slip of the finger? Masochist? It's not yet known. But over the past two days, an unknown wallet holder has paid some $5.2 million in transaction fees for two ether transactions.

Just before 10:00 UTC Wednesday, an unknown wallet holder sent 0.55 ether (ETH) (around $133) with a 10,666 ETH transaction fee – currently worth just under $2.6 million.

The fee went to Chinese mining group Spark Pool, which processed the transaction and may distribute the millions to its members. A normal transaction fee would likely be up to $0.50 or so, but can be manually set higher by a sender if they want to push a transaction through more quickly.

Around 4:00 UTC Thursday, the same address sent out another 350 ether (ETH) with another 10,668 ETH – worth $2.6 million – in transaction fees. The block was mined by the Ethermine pool.

The identities of both the sender and receiver are not yet known. But the sender has an ETH wallet balance worth more than $11 million – even after spending $2.6 million in transaction fees. In comparison, the receiver's wallet is now empty, with past funds transferred out to other wallets.

Strangely, the sender's wallet has been sending out transactions every minute in recent hours and with attached fees worth less than a dollar. As such, it's possible the whale accidentally reversed the figures for this odd transaction.

See also: Bitcoin Transaction Fees Decline as Network Congestion Eases

This isn't the first time Spark Pool has been on the receiving end of a transaction fee windfall.

Last year, the company froze a mysterious 2,100 ETH payment (then worth $300,000) it made for mining just one block – 600 times the average block reward at the time. After tracking down the sender, a South Korean blockchain firm, Spark Pool agreed to split the reward 50/50.

Regarding this latest transaction, a Spark Pool spokesperson told CoinDesk it was following up and welcomed any potential leads about the identity of the sender just "in case it was sent by mistake."

Following the publication of this article on June 10, the spokesperson said it had frozen the payout to miners in its pool.

UPDATE (June 11, 2020: 18:17 UTC): This article has been updated with news of a second transfer from the same address, with another transaction fee worth $2.6 million.