The idea I had involved a set of conjoined twins (yeah, I know, I just gave the ending away, but really...it's only a few paragraphs long, so who cares), one of which is criminally insane. I could have easily stretched this out into a long short story, or even a novella, but I thought the briefness of it made it more creepy.
The good people over at Death Throes Webzine http://www.deaththroes.net/ were kind enough to publish my short short piece on their blog. They have a great web magazine with always changing fiction thats well worth a read.
***(The magazine lost it's blog where this story was published during a re-design. I don't know when or if they will be getting it back intact)***