We have a club member here in the Northern Chicago suburbs who did a similar experiment and produced 5 gallons of beer with a "spontaneous" yeast cultured from a dish of wort left in the backyard. The results were interesting. I asked a friend who is a professor of microbiology about this type of experiment and she advised that there are a number of pathogenic organisms that can ferment a sugary liquid besides yeast. Her basic advice was that it might simply be yeast, but it also could be something that could make you very sick or even kill you. If it's somewhere in between, you might serve the beer with a caveat that anyone who has a somewhat compromised immune system might not want to try your "experimental" brew. I have decided that its pretty difficult asking people about the status of their immune system when I am serving them beer.
I tend to be more cautious with these types of experiments, and use commercial cultures for all of my sour beer experiments simply because I feel safer knowing exactly what is in my beer.