title_body: What influence do the dry residuals of previous coffee in the cup have on the drinking experience?
upvoted_answer: You're actually asking two different questions. 
  
  The minute amount of coffee left to evaporate in the bottom of a cup will have completely oxidized and lost virtually all of its volatile compounds (i.e. It probably wouldn't taste very good if you reconstituted it on its own). So when you add more fresh coffee to that cup, yes, you are essentially adding a minute amount of an external (and somewhat off) flavor component to your brew.
  
  Now whether that flavor component contributes to any any perceptible change in flavor is highly debatable. Line up 100 coffee drinkers in a blind tasting, and I suspect there would be no statistically significant detection between "100% fresh" and "Christian's 0.083% dust-like coffee residuals."
  
  Your analogy to washing tea cans in a dishwasher is a bit off. Washing containers in a dishwasher is generally known to leave a perceptible taste of soap on the container. Soap is completely foreign to the coffee-tasking experience so it is easily tasted if it is present. But the residuals you are leaving behind in your unwashed-coffee-cup study are still essentially coffee (somewhat off coffee, but coffee nonetheless). So mixing the equivalent of "a few drops" of off coffee in a hot cup of fresh I suspect will have no perceptible effect on taste. 
  
  I also suspect that your coffee cup experiment is not evolving over the years. The metallic cup is likely stainless steel or perhaps aluminum (and most coffee cups are made of ceramic). In either case, the cup is largely stainless, so you are not likely building up any patina&hellip; so you are essentially "washing away" the previous day's experiment each time you add a fresh cup of hot coffee. There should be no substantial, cumulative effect.