The Counterparty project (similar to Mastercoin) uses "XCP" as its currency. The only exchange that deals with XCP is www.poloniex.com

Last week, Poloniex users have had 115 BTC stolen

The story is (supposedly) someone found a bug in the Counterparty protocol and was able to double spend 35K XCP for 115 BTC, and withdrew the BTC.

Here is the 114.9996 BTC stolen:  https://blockchain.info/address/134V1Rt8tBtz7RuydzBtvfuNNfyHmDCHSp

Details are here:  https://bitcointalk.org/index.php?topic=395761.msg5307430#msg5307430