The blockchain allows a small amount of text to be added to each transaction, and someone took advantage of this to upload the signature for the virus DOS/STONED into the blockchain.

The DOS/STONED virus is a prank virus which doesn't do any actual damage. All it does is display "YOUR COMPUTER HAS BEEN STONED" on one out of every eight boot-ups. The virus is over a quarter century old, and may not even be compatible with modern operating systems.

Because it's supposedly only the signature of the virus, and not the virus itself, there is no likely danger to Bitcoin users. But this concept does represent a dangerous precedent. Even if it's only the virus signature, the fact that malicious code can be inserted into the blockchain could cause fear and uncertainty in the blockchain, and push users away from Bitcoin.