I helped a friend brew last night using extract and we didn't take the pot off the burner (we did a full boil) and we noticed black stuff floating around every once in a while. We came to the conclusion that it was extract that had scorched on the bottom, even though he stirred the whole time while he added the LME. Damn...
Make sure, if you are using extract, you take the pot off the burner when you add it. Electric stoves are notorious for darkening wort and scortching extract (at least in my experience and when i've talked with some people at my LHBS).
I didn't think it'd add any adverse flavors, we got all the black floaties that we could out of there.