During a brief stint at a hospital I coded some logic to check for medication interactions with procedures and anticoagulants. Then the head of the pharmacy department showed me a report showing me the number of times that catheter / anticoagulant interactions had been caught because of my code (apparently this is less intuitive to the system than base medication interactions). Was one of those, oh wow, I'm actually helping people moments.

Almost makes up for when some of my software was used to identify a manufacturing problem with a particular facility producing parts for missiles. Sometimes doing a good job helps people, sometimes it helps to harm them. Realizing the larger impact of your code can hit hard both ways.