As an intern, I was told to do a cognitive assessment on a patient. Old guy. He was obviously demented but my attending gave me an order so... I'm asking him attention, memory, language questions and he becomes frustrated. He starts crying "you're killing me. you're killing me." 

I figured I could give him a break and come back after lunch to finish the little we had left. I go to lunch and come back to his room to find an empty bed. I asked the nurse "Where's Mr. X?" She told me, "he died."

To this day, 20 years later, it still spooks me and makes me a bit sad.