Nothing at stake (NoS) is an issue emerging in the distributed consensus concept Proof of Stake (PoS). If 2 individuals (forgers) are both meeting the criteria to validate a block at the exact same time, it results in a fork. A fork means that two identical blocks are being created, both containing identical information of the original block. If this happens, it is possible that 99% of all other forgers turn to both blocks, as they just want maximize transaction fees. In this situation the 1% could possibly double spend its stakes as it could add the transaction to both blocks.