I like fermenting on-top of the previous batch's yeast cake. But on the other hand, with all the care we put into cleaning and sanitizing, I'm worried about contaminating the new batch. I rack them to kegs on brew day, but my primaries will have been going for over a month when that day arrives. One of them will be a thirdy, and the insides are plastered to the rim with yeast residue. No worries, or am I doing something wrong?