The 2012 attack on the cryptocurrency CoiledCoin by a large mining pool called Eligius, which stopped new transactions from occurring and caused the closure of the cryptocurrency, is theorized to be an example of a 51% Attack.

2012 attack on CoiledCoin cryptocurrency

- 2012 attack on CoiledCoin cryptocurrency

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When a miner has over 50% market share in a transaction, they can double spend bitcoin by replacing an initial transaction with an alternative one through the miner's greater processing power. In many cases, a 51% attack relies on replacing the public history of the transaction with a version favored by the attacker.

51% Attack

 Deuflhard, Florian and Heller, C.-Philipp, Antitrust Economics of Cryptocurrency Mining (September 3, 2021). Available at SSRN: https://ssrn.com/abstract=3917012 or http://dx.doi.org/10.2139/ssrn.3917012 

Antitrust Economics of Cryptocurrency Mining

Newer cryptocurrencies have been shown to be vulnerable to a 51% attack. A cryptocurrency with a lower hashrate could be overwhelmed by a miner who adds blocks quickly. The attacker could then direct the users of the cryptocurrency to their own cryptocurrency. In some cases, even after a cryptocurency declines in value after a 51% attack an attacker can gain short positions of the cryptocurrency while the depreciation in value occurs.

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