What really really shits me is rude people or people that give attitude to others for no reason. I really dislike that. It is just utter garbage to mistreat someone, even if they don't know them, because their garbage self feels too entitled to talk to someone with respect.
What also annoys me is people publicly humiliating staff members in a store or a counter who have nothing to do with the person's problem all because something went wrong with their product/service. Again, no need to be rude to the person trying to help.
Other than those two probably obvious things, a left field one has to be some dog owners.
I have an extreme dog phobia and I just don't feel comfortable with a little lady walking outside holding 3 massive greyhounds, rottweilers or pitbulls. That's just a noooopppppeeee for me. I literally either start walking on the road way before those dogs are near me or I cross the road all together.
The worst would have to be seeing a dog without a leash. Now that, that is something entirely different. A different level of anxiety and stress.