What is a phrase or idiom that you’ve been getting wrong for the longest time?

Mine just happened two days ago. I was reading an article, and the author described someone as a “fount of knowledge” about a particular subject. I thought that it must be a typo, and they meant “fountain of knowledge.” However, I realized how stupid I can be sometimes, so I looked up the word fount. Yep, a fount of knowledge is someone who knows a lot about a specific subject. These are not the fountains you’re looking for.