This is Monero's second successful 51% attack.

Bitcoin has no 2nd best.

"What happened technically: An 18-block "reorg" (reorganization) hit the Monero network - meaning that 18 blocks the entire network believed to be valid and permanent were suddenly discarded and replaced by an alternative version of history. To put this in perspective, reorganizations of more than 2-3 blocks are extremely rare in established blockchain networks.

The financial outcome: 55 confirmed double spends (the same money spent twice) and 115 transactions completely invalidated. While we cannot know whether these were merchants, exchanges, or individuals transacting, this matters little in the face of real double spending scenarios - someone lost real money."