The scenario that I would find most challenging would be to withdraw life-sustaining treatment for a child. Ethically, it would be a dilemma between non-maleficence and beneficence. In situations like these, it is often the right thing to do for the child, and not to prolong their suffering. However, it feels like an act of maleficence because you are actively withdrawing something that is keeping them alive.
In my work place, we sometime get patients that come in too late, and we cannot do aything for them to reverse their illness. We just have to make them comfortable and counsel their family. It is very difficult to feel so helpless, and to watch someone die, knowing that there is nothing that you can do do help them.