Some authors[15][16] argue that proof-of-stake is not an ideal option for a distributed consensus protocol. One problem is usually called the "nothing at stake" problem, where (in the case of a consensus failure) block-generators have nothing to lose by voting for multiple blockchain-histories, which prevents the consensus from ever resolving. Because there is little cost in working on several chains (unlike in proof-of-work systems), anyone can abuse this problem to attempt to double-spend (in case of blockchain reorganization) "for free".[17]

Many have attempted to solve these problems:

    Peercoin uses centrally broadcast checkpoints (signed under the developer's private key). No blockchain reorganization is allowed deeper than the last known checkpoints. The tradeoff is that the developer is the central authority controlling the blockchain.
    Nxt's protocol only allows to reorganize last 720 blocks.[18] However, this only rescales the problem: a client may follow a fork of 721 blocks, regardless of whether it is the tallest blockchain, preventing consensus.
    Ethereum's suggested Slasher protocol allows users to "punish" the cheater, who mines on the top of more than one blockchain branch.[19] This proposal assumes you must double-sign to create a fork and that you can be punished if you create a fork while not having stake. However Slasher was never adopted; Ethereum developers concluded proof-of-stake is "non-trivial".[20] Instead Ethereum designed a proof-of-work algorithm named Ethash.[21]
    Hybrid "Proof of burn" and proof of stake. Proof of burn blocks act as checkpoints, have higher rewards, are more secure - and are more expensive.
    Hybrid "Proof of work" and proof of stake. Proof of work blocks also act as checkpoints, containing no transactions, but anchor both to each other and to the PoS chain.

Statistical simulations have shown that simultaneous forging on several chains is possible, even profitable. But Proof of Stake advocates believe most described attack scenarios are impossible or so unpredictable that they are only theoretical.[22][23]