“The Beatles? Wasn’t that Paul McCartney’s old group?” (high school, ~1983)
“Plutonium? Isn’t that the stuff the planet Pluto is made of?” (in Chemistry class)
“Is the planet Mercury made of the element mercury?” (in Chemistry class)
“Is this Algebra Two or Algebra Eleven?” (to a colleague)
“Do you ever get cysts on your ovaries?” (to me)
“Did you get a haircut?” (innumerable times)
There are, of course, many other examples. Feel free to leave your favorites in a comment.