In the airlock that is. I have used the 3 piece every time and never had this problem before. I just brewed an IPA today and had about 3.5 gallons at the end of the boil. When it was cooled to high 80's or so I poured it on top of the cake from my stout that I kegged today. I topped off to about 5.25 gallons like usual but what I did different was I placed the Carboy in a water bath immediately. Usually I wait until the next day to let the yeast get a head start before I drop them down to mid 60's. But since I was pitching with an entire yeast cake I knew it is going to take off fast so decided to drop the temp right away. Anyway I guess the temp in the carboy was dropping too fast making negative pressure in the carboy which sucked the water from my airlock into my brew. I did starsans the airlock and used a brand new bottle of drinking water which I hope is mostly sterile. I hope it works out. Odds are it will be fine but it was pretty disconcerting to see the airlock bubbling backwards
