Your hard earned groats might be worthless folks http://arstechnica.com/security/2014/06/bitcoin-security-guarantee-shattered-by-anonymous-miner-with-51-network-power/
Researchers from Cornell University say that on multiple occasions, a single mining pool repeatedly contributed more than 51 percent of Bitcoin's total cryptographic hashing output for spans as long as 12 hours. During these periods, the GHash operators had unprecedented powers that circumvented the decentralization that is often held up as a salient advantage Bitcoin has over traditional currencies.
So-called 51 percenters, for instance, have the ability to spend the same coins twice, reject competing miners' transactions, or extort higher fees from people with large holdings. Even worse, a malicious player with a majority holding could wage a denial-of-service attack against the entire Bitcoin network.
The skies falling in !!!
Thankfully the reality is that no one has yet seen any of the malicious activity that's "possible" but in the main very unlikely, the solution of course is for those concerned to get mining on an industrial scale and thus make it virtually impossible for anyone to achieve the 51% ratio.