Just come it to my mind. is it possible? to get hook into your wallet and send some to another wallet? 
I don't know if you have seen but last months i have seen that many people were infected by a virus or malware which when people wanted to make a transaction the pasted address was always the hackers address so people were sending accidentally money to the hacker.
Well its the first time i see someone talking about some malware virus that affects the adress where we wanna to send the bitcoins,besides those i believe currently there is no way to hack bitcoin,since the hash protects the network.
Well only smart hackers did/do this and it had affected lots of people because it's hard to remember the address especially when you paste it directly when making transactions. That's why it is important to run your bitcoin client in a safe environment.