One of the most notorious threats to blockchain-powered networks is allowing a malicious actor to obtain control over more than half of the network's computing resources. This situation is commonly referred to as the elusive "fifty-one percent attack." It is difficult, but not impossible to pull off —http://motherboard.vice.com/blog/bitcoins-fatal-flaw-was-nea... — the aggregation of this kind of hashing power. And it looks like we've nearly made it there again, but there is a difference between a pool getting > 50% of resources, and an "attack."
A successful attack using this method would allow the attacker to exclude, or "orphan" any new blocks from the valid chain causing all newly-minted coins go to the attacker. He may also execute a "double-spend" and reverse any of his own transactions during the window of time that his sham blockchain is considered authoritative by the network.
A successful 51% attack on Feathercoin — http://www.coindesk.com/feathercoin-hit-by-massive-attack/ — was stopped in its tracks by the network's natural uptick in difficulty in response to an increase in network hash rate. The Feathercoin attacker was likely executing a price pump in parallel to compromising the network, which caused coin-switching pools to mine feathercoin and increase the difficulty to a level that stopped the attack.
[0] GHash.io homepage
GH just did a great job, I think when other competitors step up and provide as good a service - this will become a distant memory of the past - a blip on the radar (I hope).. There'll be a handful of decent pools, thereby never letting this happen again. (I hope again)
