I've recently had a discussion with another brewer that concerned me. He was talking about how you can get off flavors from fermenting with too much head space in the primary. For instance, only filling the 6.5 gallon carboy with 3 gallons of wort. He said it has to do with negative pressure affecting the beer. Do anyone know anything about this or is it just the rantings of a mad man? I'm curious as to this because I usually just make 3 gallon batches and had used a 6.5 gallon plastic bucket for fermentation. There seems to be an odd taste to it that I had been trying to figure out.