At work I usually drink coffee (multiple times daily) from a metallic cup which I actually never ever wash on the inside. This leads to the building of interesting dust-like coffee residuals. While this started out of mere laziness, I meanwhile deliberately refrain from washing it as the result is visually quite appealing and an interesting long-term endeavour to see how that coffee cup evolves over the years.
I am however wondering if those residuals actually have any kind of influence on the fresh coffee drank from this cup. Now of course there isn't anything moulding, it's an entirely dry thin dust layer, and I don't really make out any taste difference to a clean cup (as the coffee isn't too much of a high quality product in the first place and I'm not a big tasting expert either). So this might be a stupid question. But for example I heard somewhere (though, that's entirely anecdotal hearsay, too) that you shouldn't wash a tea can with dishwasher but only with water because of some flavoury reasons. So maybe there is actual proper or even scientifically grounded reasoning that the dry residuals of previous coffee in the cup have any kind of influence on the coffee drinking experience, however small and imperceptible, be that flavour-wise or regarding other effects, advantageous or defective.