Nothing-at-stake problem

The nothing-at-stake problem is a theoretical security hole in proof-of-stake systems. The problem can occur anytime there is a fork in the blockchain, either because of a malicious action or accidentally when two honest validators propose blocks simultaneously.

Whenever a fork occurs, it's in the best self-interest of all of the miners to continue mining both chains. There are two reasons for this:

This could potentially make double-spend attacks more feasible. An attacker who wants to attempt a double-spend could do so by creating a fork in the blockchain one block before they spent some coins. If the attacker only mines their fork while all of the other miners act in their best self-interest and mine both forks, then the attacker's fork would eventually become the longest chain even if the attacker only has a small stake in the network.

Although the nothing-at-stake problem hasn't yet occurred in the real world, it is still an issue that EthereumEthereum developers are addressing in their PoS protocol, Casper.