I was called in to clean up a virus infestation at a local bank. The office
manager had a sound security policy in place: only two PCs in the bank
had Internet access, and only certain staff were allowed access to these
machines, and only for bank business. The internal LAN was not
connected to the Internet at all.
So in a dazzling display of ingenuity, the bank president brought in a
floppy disk containing a file infected with the SirCam virus. He could
not open it on his home PC, because his anti-virus program wouldn't
let him. I am not making this up. So he brought it to work and infected
the entire LAN. Wotta genius.