What Is the ‘Nothing at Stake’ Problem Unique to Some PoS Systems?

The 'nothing at stake' problem arises in some pure Proof-of-Stake (PoS) systems where validators have no financial incentive to choose only one chain in the event of a fork. Since validating on multiple chains costs them almost nothing, they might sign blocks on all competing forks to maximize their potential reward.

This behavior undermines the security of the consensus mechanism and makes it harder for the network to reach a single, canonical chain.