A scary story for Halloween.
This happened about 22 years ago, and it's one of the scariest things that a computer ever did to me.
I was working in the Virus Lab, which was actually a small bedroom being used as an office, with strict rules about what could leave the room. I was working late at night, which I often did, partly because that's a good time for being able to concentrate free of distractions, and partly because there was just so much work to do. To maintain Findvirus, I had to replicate each file that came in as a possible virus (a large number of "suspicious files" weren't viruses, and I even wrote a program called "dustbin" to recognise the checksum of files that I'd already checked and decided weren't viruses). Then after replicating it, and getting it to infect several of my standard, very simple "sacrificial goat" files, I could then disassemble the goat file, knowing that everything I saw there was part of the virus, and not part of some complex host program.
I used two computers (at least). One was for actually running the virus, this was the "goat" computer, called "Dobbin", and the other was my programming machine, used for disassembly of the virus and for writing the Virtran (virus transaction language) that would describe how to find the virus, what to look for and exactly where, the checksum of the static code to do an exact identification (which is necessary before attempting a repair) and then the steps necessary to strip the virus out of the infected file and restore it to how it was before it got infected. I also had a computer used for timing tests, so that I could check that Findvirus was still twice as fast as the nearest competitor.
So I'd got the virus to infect the goat files on Dobbin, and I copied the goats onto a floppy (no network in the virus lab) and started work on the disassembly, which is best done with complete concentration.
Suddenly, I started hearing this music. I nearly jumped out of my skin. It was coming from Dobbin, being played via the little speaker that was what all computers had, used for making the beep noise, but you could also play music through it (but not very well).
It was, of course, the virus that I'd left running on Dobbin. It was several minutes before I could calm down enough to get back to work.