A recent 51% attack on a PoS coin.
https://bitcointalk.org/index.php?topic=483847.0

Granted it is a small coin, from a clueless developer but it does illustrate one common misconception.  A common argument from PoS supporters is that obtaining 51% of the money supply would be nearly impossible however that isn't the requirement.  The attacker only needs 51% of the network stake which will be some fraction of the total money supply.  In the case of "Coin2" it looks like the money supply is ~60M NC2, and the active network stake was ~10M NC2.  It only required the attacker to obtain >10M NC2 (~16% of the money supply) to attack the network and that assumes the 10M network stake didn't contain any of the attackers coins.   The attacker can make the network appear more secure than it is by adding to the network stake prior to the attack.  We don't know exactly how much of the 10M network stake was held by the attacker but lets say it was 6M NC2 that means the effective security was only 4M coins (~6% of the money supply).  

Once the attacker had more than 51% of the network stake, he executed an a double spend against mintpal (an exchange) resulting in a loss for the exchange (customers) of 22M NC2.  

The network stake will never be more than a fraction of the total money supply as coins used for staking are essentially locked capital.  A coin with 100% of the money supply being used as a stake would require 100% of the coins to be in hot wallets not being used for anything else (no cold storage, no transactions, no economic activity).   Looking at other PoS coins the network stake tends to be somewhere in the range of 20% to 30% of the money supply.  

The "fix" from the developers is a centralized seizure of the network and reboot.  While that can "work" for a small pump and dump altcoin with no future it obviously is not viable for any crypto currency to be taken seriously.  Centralized security for a decentralized network is an oxymoron.