High viscosity fermantation I recently tried an experiment with using strong dandelion flower &amp; stem infusion and malt syrup. I was unable to obtain or culture a yeast starter in sufficient time so the wort sat for approx a week and started to spontaneously ferment in that time.  I assumed we'd end up with sour ale or something along those lines but unfortunately the brew became highly viscous - almost syrupy.  My best guess is something other than yeast got busy inside the carboy and maybe went to work on the proteins or whatever else came from the dandelion.

I tasted it and it was a bit sour - not unpleasant other than the fact that it's so thick makes it a bit disgusting.  Does anyone have any idea what organism, specifically, would cause such a change and turn liquid into a thick substance like this?
Well, a bacterium, of course... But there are quite a few bacteria which will thicken the ferment (some of them benign and some harmful). Without in-depth testing it would be impossible to know which ones were in your sample. (If I had to guess I'd say acetobacter.)