1) You buy old keys that at the same time in the blockchain history had 15% of all coins. Maybe some old exchange keys (BTC-E? 
) will do the trick. (That is the "bribing attacker" Vitalik Buterin has described in his PoS analysis.)
2) You buy 15% of the supply and send them to your wallet, send them back to the exchange, privately mine a double-spend attack chain, and sell the coins again, and then you publish the chain for an attack.
) will do the trick. (That is the "bribing attacker" Vitalik Buterin has described in his PoS analysis.)2) You buy 15% of the supply and send them to your wallet, send them back to the exchange, privately mine a double-spend attack chain, and sell the coins again, and then you publish the chain for an attack.
3) You convince a majority of coin stake* to send a transaction to themselves at the same time and you bring the entire network to a semi-permanent halt as no new blocks will be produced since PoS coins are forced to put safeguards around the amount of time taken after a transfer of funds before staking can begin due to other long range attacks.
There are so many variations of the long-range attack that we almost certainly haven't discovered them all.
Cheers, Paul.
*) this could be as little as three people, depending on the distribution of coins at the time