Yeah, it's happened a few times. Mostly basic stuff like helping someone who tripped and fell, or checking on someone who seemed like they were having a medical issue in public. You kind of just switch into "EMT mode" without even thinking about it.
My first 911 call was actually a false alarm. I was home alone as a kid and thought I heard someone trying to break in. Turns out it was just the wind rattling the windows, but I was terrified!