We had an 'office boy', a Mozambican, in our Cape Town office whose job was to make tea and coffee and run errands. We noticed that the tea and coffee became progressively worse tasting and more evil smelling as the week went on. The girls sent me to ask him how he made the drinks and if he washed the cups.
"Oh yes", he said, and took me out to the back where, next to the kitchen sink, was a plastic bowl full of black liquid with a rag submerged in its depths. I recognised the rag. It was the same one that he used to clean the floors, wipe the desks, and of course, wash the cups with. He told me he changed the water every week.