The learning experience, and there's always one, came the following morning in this particular batch. We are using a corny keg as primary fermentation, and filled it to within about 4" from the top, propped the legs of the seal and closed her up. Using the liquid quick-disconnect fitting and a length of hose leading into a 1/4 full bottle of cheap vodka, constructed a larger than normal pressure lock.
The yeast started in on the wort almost immediately and the next morning the vodka bottle was half full of light brown liquid and yeasty foam. Called my local beer supply and our buddy, Kel, told me we shouldn't fill a 5 gal fermentation vessel. "You need to leave about 8" for the yeast break," he said. "So to do 5 gallons, you really need a 6.5 gallon container." He said we would lose a lot of beer and possibly all our hardworking yeast bodies to the current setup, and recommended a chemical product called Kettle Defoamer. We picked up a bottle of the stuff, which resembled Elmer's Glue in color and texture, for $6 and set about to Save the Beer.
The yeast had produced so much gas that the fermentation lock was bubbling nonstop, the only outlet to our well-sealed keg. This made it impossible to open the top of the keg, and if I had paid more attention to high school chemistry I could probably calculate the pressure holding the lid in place, but suffice to say our 180# roommate could not make the sucker budge. We added pressure to the quick-disconnect fitting to allow more gas to escape, along with a pile of foam which condensed to about a pint of liquid. Soon we had released enough pressure to open the top and deposit 10 drops of the defoamer, but the foam was unstoppable and rose up out of the keg, taking the goopy substance with it. Threw a few more drops in for good measure and closed up the top before more foam could escape. The foam subsided almost immediately and the pressure lock water has remained clear since.