I suppose what I've seen that's most likely to lead to patient harm is the inappropriate use of antibiotics for viral conditions or where there's no infection in the first place, or where the wrong antibiotics have been chosen and that leads to cases of C. difficile. C. difficile, especially in the elderly, can be very difficult to treat and can require prolonged hospitalizations, or as your colleague said, can lead to death.
On June 10th, 2014. See this statement in context.