What is so annoying about being gullible is the shock I get when theists say something that is so silly, I simply do not believe them.
One friend told me he didn't believe in science, specifically that the universe is 14+ billions of years old because the scientists who make/use that equipment... lie. How can they measure it? How do you measure radioactive decay in dinosaur fossils? You can't... they lie. I thought he was playing a joke on me! When I figured out he wasn't kidding, I was taken aback! How, in this day and age can someone think this way? I don't care if it conflicts with your beliefs, how do you deny such well established facts?
This kind of denial is what gets me above everything else.