What Is the ‘Nothing at Stake’ Problem Unique to Proof-of-Stake?

The 'nothing at stake' problem arises because in PoS, validators can vote on multiple competing chain histories without significant cost. Unlike PoW, where mining on multiple chains requires duplicating computational work, PoS validators risk nothing extra by validating conflicting blocks.

This could theoretically hinder the network's ability to achieve consensus after a fork. Modern PoS protocols use 'slashing' to penalize this behavior.