My boss at work. Am I over-reacting?

It's not abnormal to be upset about this. Just understand that not all bosses are equal. Some are very short-tempered and have absolutely no patience. These types of people should never be put in the positions that they are in as managers, and some even look for things to pick at people about because they know it bothers them and the boss gets a sick thrill out of putting others down. I used to make mistakes printing out stuff at my old job, and our boss knew we really didn't mean to do it just as long as we didn't constantly do it and waste tons of paper all the time. I would say just take your time on your job so maybe you can catch mistakes before they make the final copy. Try to kill your boss with kindness even if you can't stand him. I used to have a boss that was dissatisfied with my speed of work and thinking and was always getting impatient, but I know she still likes me because I tried my best and she still asks about me.