Can a Double-Spend Attack Occur without a 51 Percent Attack?

Yes, a double-spend attack can occur without controlling 51 percent of the hashrate, though these methods are generally less reliable and harder to execute. Examples include the "race attack" and the "Finney attack." A race attack involves broadcasting two conflicting transactions simultaneously to different nodes, hoping one confirms before the other.

A Finney attack involves a miner pre-mining a block with a transaction that is then reversed on the public chain. However, a 51 percent attack is the most powerful and reliable method for a large-scale, sustained double-spend.