Cigarette butts. I'm HORRID about people who just toss them out, still lit and all. I have absolutely no self control when it comes to seeing someone do that. I don't yell, but I very sweetly stop the person and say "Excuse me, I think you dropped your cigarette." Then when they give me the brush off I say, "I would hate to think that some small child would pick it up. Or that a bird would use it as nesting material and start a house fire."
At the least, they usually stamp it out then.
But that's a regular thing for me. As to random, surprising even yourself...
One time, we were in a grocery store and there was a man and woman shopping. They had their toddler with them. He was just CRYING his eyes out. This was clearly not a temper tantrum, but a real, honest upset for the child. Mom and Dad were just continuing shopping and periodically yelling "HEY, knock it off!" to the kid.
Unable to take it, I went to the cart to see what was wrong. The poor thing had his fingers pinched in the cart!
I let rip at the parents (AFTER freeing the little guy's fingers). I yelled at them that if they weren't too absorbed in which potato chips to buy, maybe they'd notice that their son was IN PAIN.
It was actually one of the ONLY times in my life I've ever cussed at anyone!
I was so shocked after! I was actually shaking like a leaf by the time I got back to the car.