There's a flaw in the way the Bitcoin protocol distributes objects which can be used cause mischief with a PoS (and hybrid PoS/PoW) coin that has low PoS difficulty. I won't go into further detail, other than to say I have discussed it at length with Sunny King and for a popular coin like PPC is unlikely to be possible, but for the quieter coins it is of more concern. I can't see any easy way to fix it.