I thought Full Metal Jacket had something to do with Fullmetal Alchemist. I thought it might have been some kind of silly spinoff.
A childhood belief about cartoons

others believed things like this
- I used to believe the large metal structure in Paris, France, I thought it was the "Eye Full Tower"— Janet
- I used to believe that Metal music was when a bunch of people would bang metal together to make noise. Ha ha.— Kassandra
- When I was a child, I thought that the Heavy Metal was Satanic music.Anon
- When I was a little I thought TV was full of tiny books and we would watch one every day— Amine
- Before I went to a planetarium for the first time, I thought it was just a room full of plants. Anon
- When I was little, I thought that Water Towers were full of dirty laundry. Why? I dont know.Anon