Hot beverages, and how to ruin them.

werehatrack thought this was worth mentioning said

Number one on my personal list of gripes about hot beverages outside of those I make at home is the certainty that any given coffeemaker in a hotel (and many offices) will have been forever made useless for the brewing of tea or the heating of water for cocoa by having had already-brewed coffee poured back into it to reheat. Second place goes to the restaurants that brew tea in their coffeemaker using the same basket in which they’ve brewed coffee.

What ways have you found that others have ruined hot beverages, aside from the obvious “use the wrong amount or use crap ingredients”?