15 years of Fire/Rescue/EMS lots of things...
Honestly, the ones that are obviously dead when I get there bother me, but only somewhat. The early fatal scenes stick with me a little more. I regularly drive by a cross on the side of the road that marks the first fatal I ever responded to. I think from time to time about seeing a young man that was almost my same age, with his neck snapped from hitting a utility pole at high speed. The shame of it was, the pole was on the edge of a cornfield. If he hadn't hit it just the way he did, he would have walked (maybe even driven) away fine. There have been many more gruesome, such as industrial accidents, pedestrian versus vehicle, train, fire, long falls, but that was one I always remember.
The ones that really bother me, are the ones that I found alive, but mortally injured. Working my best to save them, talk to them, sometimes listening to their begging "Please don't let me die!" Unfortunately this has happened more than I care to remember. Despite my best efforts, and close access to the home of trauma medicine, I have watched numerous people die in front of me. You never get used to that.
I will say that treating friends and family has also taken a toll. One of the first shooting victims I ever treated was a high school friend of mine. Mutual friends were there, again pleading to save him. That one actually turned out well, though others haven't. Another one was working a medic unit close to home, and I walked in and was greeted with "Good it's you! Mom-Mom isn't acting right." I immediately recognized symptoms of a hemorrhagic stroke, and knew there was a large chance that this event was going to be fatal. I have to put aside personal feelings and act, but those feelings don't disappear they simply are delayed.
As with many, the kids are hard to deal with. Seeing children whose death/serious injury was at the hands of their parents was particularly hard. I have done CPR on patients small enough to use a single finger... I will leave it at that.
All in all, I would say it's not one event, but the cumulative effects of all the deaths I have seen that live with me today. I would say at least weekly, if not daily I am reminded of one or more of those incidents.