After pitching the yeast on Wednesday night into my Franziskaner Hefeweizen clone, I set 6.5-gal bucket in the basement in a warm corner with an air lock on top. Thursday morning saw no activity. However, when I came home Thursday night, I found dried brown foam all over the airlock and the cap had blown off. It was on the floor about 2 feet away.
Cleaned this up and replaced the airlock cap. Woke up this morning to find even more foam and some liquid all over the top of the fermenter lid and the airlock cap was on the floor again. All I can guess is that the fermentation was so strong that the foam built up enough to come out of the airlock - once it did come out, it formed a seal with the airlock cap. Enough pressure built up to blow the cap off. That's the first time I've had that happen, but apparently I'm not the only one to experience this with a hefeweizen. At least I know it's fermenting!