Warren -
No, actually, it was pre-fermentation. I tasted the wort in the morning from the tun and it was sour before it went into the kettle. I went ahead and fermented anyway just to see if I could salvage it, but it didn't work.
And, FYI, I stopped using water from the toilet after I saw my dog drinking out of it... I didn't want cooties... :blink:
You'd probably be surprised at the levels of various microbes in piping -- the water may come from a pure\purified\treated source, but the road it takes to your tap is far less controlled. I used to live in New York City (which has some VERY clean water and extremely strict regulation of water quality), and my water was AWFUL. It turned out that the 180 year-old building in which I lived had only had the pipes replaced once -- in 1927. 75 years of buildup was icky, to say the least.
I currently live in Mexico and any water that I leave standing around develops an incredible sulfobacter and acetobacter infection (I mean, you can smell this shit from about five meters away!). So... yes, it IS possible (and probable, actually) that unboiled water from old pipes, when mixed with a nearly perfect culturing medium and left overnight at ideal incubation temperatures will carry enough microbes to innoculate your wort.
As to the mash not sanitizing the wort, thats obvious! However, most people strike with water in the 75-90C range -- water that has, for all effective purposes, been sanitized (unless your water comes from an archaebacter laden hot-spring, in which case you've got other problems) and which will NOT add a large proportion of beer-unfriendly bacteria to your mash. Most of the microbes on malt don't adversely affect your wort anyway...
Liquid decoction: why? Why not just runoff into the kettle and start your sparge (batch or fly) with new water... at this point its extremely unlikely that there will be that many sugars left over for the enzymes to attack ANYWAY: its been mashing for 8+ hours! Just start your boil with 1st runnings, sparge and trim off an extra 30 minutes! This is about saving time, no?