Ungrateful people.
Something I've always done since i was younger is hold the door for people behind me when entering or exiting a building. Especially if they have a full cart, full arms, or if they are elderly or disabled in some way. I'm admittedly not the nicest person in the world, but for some reason, I always thought this was a simple act of courtesy and it usually makes me feel good.
I'm leaving a doctor's office last week, and I encounter a VERY elderly couple shuffling in. Moving slowly, the old man opens the outside door for his wife and she struggles in. I'm at the inside foyer door so I open it and stand aside, holding it open for them both. The man stops, turns to me and says, "I don't need no one to hold a door open for me," and continues on his way. I literally stood there and had about a thousand different things I felt like saying/doing to this old bastage run through my mind, but I decided it was best to just walk away, as this feeble geriatric was obviously miserable enough, I wasn't about to get into it with him. Unbelievable.