History blindness.

 There are people who lack a sense of history. I'm not even talking about a lack of education: somebody who only watches TV can still grasp that there was a time before TV when people had to watch plays instead. I'm talking about people who don't seem to grasp that an era ended before a different era began.

Here are examples that I have observed from just talking to people. These are adults who are at least mentally sound enough to have gainful employment, and did not seem at all sarcastic when they said what they said.

1. I joked to a coworker that a different coworker was so old that she had a Confederate birth certificate. He looked at me and said "Really?" as if I was telling him a weird fact instead of a joke. A high school graduate should know when the Civil War happened and the limits of a human lifespan.

2. I was chatting with a coworker who was a Vietnam veteran. He told that he shook hands with a general and got rifle grease on his hands, and the general just laughed it off. I asked him if it was General Westmoreland. The vet confirmed that it was Westmoreland. Another coworker overhears this, looks at me, and asks "How old are you?" with a confused look on his face.

3. At a party, I was talking about a place that I used to work at. At that location, uranium was rolled into bars for the Manhattan Project, and workers had no idea that these bars were being used to develop the first atomic bombs. A woman asked "But you didn't have anything to do with that, right?"

4. A friend of mine was telling me about a guy in her gaming group. She said that he was in his 50's and had served in World War 2. This is especially egregious because I met her grandfather who raised her and had actually served in the Battle of the Bulge.