I heard on the last Sunday Session about how a Helles Lager obtained it's smoky character from the previously brewed batch and I can confirm that has also just recently happened to me after brewing a decoction mash.
(Suck it!) JP mentioned how he has never had that happen on his batches. I never really noticed that happen myself until I just recently brewed a Dunkelweizen and did a decoction mash of which I accidentally scorched the decoction mash when I was ramping up the temp to boil.
My very next batch (Cream Ale) and the next after that (IPA) both had a residual "burnt" flavor from the equipment. I use a cooler for my MLT and it also has a CPVC manifold, and of course the plastic bucket for primary. Perhaps these porous plastic items carried some of the flavors into the next batches.
Has anyone else noticed this or something similar on their plastic MLT or fermenters? I wouldn't think this could happen on a system with a stainless MLT and false bottom, unless you filter (as was mentioned in the interview).