On May 15, 2014, a virus signature of an antique virus called DOS/STONED was inserted into the block chain. This caused Microsoft Security Essentials to recognize the block chain as the virus, prompting it to remove the file in question, and subsequently forcing the node to reload the block chain from that point, continuing the cycle.
Stoned was a virus created by an unidentified student from Wellington, New Zealand in 1987. It was one of the first computer viruses, and had become common and widespread by the 1990s. When an infected computer started, there was a one in eight probability that it would declare "Your PC is now Stoned!" The phrase is found in infected boot sectors of infected floppy disks and master boot records of infected hard disks along with the phrase "Legalise Marijuana".
Only the signature of the virus had been inserted into the blockchain; the virus itself was not there, and if it were, it would not be able to function.
The situation was averted shortly thereafter, as Microsoft had prevented the block chain from being recognized as Stoned. Microsoft Security Essentials did not lose the ability to detect a real instance of Stoned.