For example, I didn't know a Shirley Temple was an actual drink until last year. I was a waiter at the time and one of the tables I had tried to order it. I honestly thought they were messing with me (I had never even HEARD of a Shirley Temple before), so I gave them a blank stare and said "I'm sorry.. I have no idea what that is." Needless to say, they left a horrid tip.
So Reddit, I ask you, what pieces of common knowledge have you been totally unaware of until recently?