I just tasted a hydrometer sample from a batch of Munich Dunkel which happens to be in the same fermenter that held a sour beer made by souring and then denaturing (30 minute boil) a portion of the wort.
My dunkel is now very sour with a very sharp flavor. My first guess is that the boil did not denature the lacto, but then again, the carboy was thoroughly soaked and scrubbed, and then sanitized with star- San. Another guess is the washed yeast I used for this batch, but I am skeptical of that too since I did not smell any kind of lacto character in the rest of the yeast as I was dumping it.
I do not think it could have been infected by airborne bacteria. The carboy was covered with foil (sprayed with star-San) for the first few days, and then plugged with a blow-off tube (end submerged in jar of star-San).