I have an opaque mug and I usually use it for a week or so before bringing it home to swap out with a clean mug. It's some sort of ceramic that sort of picks up the brown of the coffee until it's washed and then it all comes off. Our coffee machine works like a soda fountain: you just press your cup against it and the coffee you made comes out from the inside.
So anyhow I had one mug of coffee and thought that it tasted weird but still went back for more. It wasn't until I was halfway through that second cup that I realized that I wasn't drinking coffee. I had forgotten to put the ground coffee into the machine so I was just drinking hot water that had picked up weak coffee flavorings from the machine and from my mug.