I'm a few days into blueberry (96oz blueberry juice, 32oz honey, D47 yeast) in a gallon juice jug. Started with with S-airlock. Never used one before. Krausen started foaming into air lock. Immediately pulled and replaced w/ 3 piece standard. Krausen bubbling into that one too. Hrm. Never experienced that before, always brewing in 5 gal batches w/ ample headroom. Did some research and it's totally a thing in gallon batches, it seems.

So, currently I have triple layer cheese cloth soaked in a starsan/water solution and rubber band holding it on. Krausen already calming down with less back pressure to contend with. Will place 3 piece airlock back on either tomorrow or Monday and see if that resolves it.

Might do a blow off tube setup for the gallon experiments.

Researching those, I found this hilarious story:
http://www.love2brew.com/Articles.asp?ID=279