As reported here, a bitcoin mining pool has contributed 51% of total hashing output which theoretically allows them to double spend bitcoins or to deny other miner's transactions. I am not an expert at bitcoins but it appears that the distributed architecture is a crucial feature in ensuring the integrity of the currency.
A similar problem exists in the Tor protocol. If a single entity controls a majority of tor exit nodes, there is a probability that that entity might be controlling all 3 nodes that the traffic is currently being routed through. As a result, that entity would be able to capture the entire packet stream, he would be able to determine the source address, the destination address and even the contents if HTTPS was not used.
The world we live in is has a great deal of inequality. 1% of the world population own nearly 50% of the wealth in the entire world. It is not surprising if 1% of the world's organisations own 50% of the world's computing power. The processing power could then be channeled into mining bitcoins or running tor exit nodes. If we continue developing distributed systems that is extremely dependent on its distributed nature to ensure security/reliability/integrity, I believe that such incidents would be more common in the near future.