I left it out for the last 7 hours and when I woke up, I noticed a small darker brown center in the middle of the cup with milk bubbles. Didn't think much of it untill after I drank it all down, heh. All seemed like regular chocolate milk going down, however. Windows where all closed, so I know it's not a bug or fly that flew in there and landed in the middle. Dang impulsivness, heh. Should have investigated it a little further, but my guess would be that it was just a result of letting the milk sit over night for 7 hours.