Personally I would just fabricate a very intelligent-sounding report where I claim to have found a serious, exploitable flaw in Bitcoin's encryption. I don't think it would be too hard to convince people, because many people expect that something like this might happen. And if the fabrication is good enough then it'll take days or weeks for researchers to fully evaluate the claim. In the meantime the uninformed panic causes a price dip and I buy a bunch of cheap coins.