1. Hiroshima and Nagasaki

What you already knew
On August 6, 1945, the United States dropped an atomic bomb on Hiroshima, Japan. That was followed up three days later by another atomic bomb being dropped on Japan, this time in Nagasaki.
What you might not know
While it is common knowledge that people survived the blasts in Hiroshima and Nagasaki, it is less widely known that some people survived both blasts. According to this
article an unknown amount of people had left Hiroshima following the first bomb and traveled to Nagasaki. One such person, Kazuko Sadamaru, had this to say about her experience:
"I cannot forget the events on 6 and 9 August 1945. I saw the flashes and the mushroom clouds of both A-bombs dropped on Hiroshima and Nagasaki. So many were exposed to the A-bomb but I am one of the few people who have experienced the two bombs, and still I am in good health. It was fate that I was there, but I had good luck in that I survived both bombs."
Good luck? It seems more likely that Sadamaru became a zombie after the first blast, and has hidden her immortality from the rest of us mortals.
2. Zapruder film
What you already knew
On November 22, 1963, John F. Kennedy was riding in the presidential limousine down the street in Dallas, Texas when he was shot and killed. A man named Abraham Zapruder was in the right place at the right time to record the assassination.
What you might not know
Abraham Zapruder almost got knocked out by this man:

That's right, Dan Rather. While working for CBS his boss
ordered him to punch Zapruder "in the mouth" and steal his film, then make a copy of it for CBS and give it back with an apology. Later that day he called the whole thing off, probably realizing how guano crazy he sounded.
3. JonestownWhat you already knew
A crazy religious cult committed mass suicide by drinking Cyanide laced flavor-aid
What you might not know
They only did that after killing a
United States Senator. Congressman Leo Ryan traveled to Jonestown to investigate the cult following significant complaints from the family of cult members. He traveled with an NBC reporter and camera man, who were also killed. After they had investigated Jonestown, they reassured the people that they would give a mostly complimentary report of what had happened. Apparently Jim Jones did not believe this, and ordered his men to follow Ryan to an airstrip and shoot him. It is the only time in US history a Congressman has died in the line of duty.
4. Men on the Moon
What you already knew
During the Apollo 11 mission, Neil Armstrong and Buzz Aldrin landed on the moon.
What you might not know
NASA landed people on the moon during Apollo 12, Apollo 14, Apollo 15, Apollo 16, and Apollo 17. Apollo 17 put men on the moon in 1972, and is currently the last manned moon mission and also was the only time a scientist (in this case a geologist) was put on the moon. Conspiracy theories claim that we never put men on the moon, but it seems to me that NASA not only put men on the moon, but more than likely were using it as a popular vacation spot for astronauts.
5. Fog
What you already knew
Fog exists, and is pretty much harmless
What you might not know
Ice Fog exists, and is pretty much harmless. Ice fog is caused by extremely low temperatures, and the water droplets in the fog get frozen in mid-air. It can also create "light pillars" in the sky, which look like this:

Ice Fog mainly occurs in arctic and antarctic regions.