Oh there's a lot.

• When I was a kid, parents and teachers used to teach, if you have sore muscles a day after an extensive workout, you need to work out even more in order to reduce the soreness. In fact, however, you need to rest those muscles.
• I thought, pepperoni was pepper. (Like bell pepper, just smaller; similar to chilli). Then my girlfriend enlightened me after a confusing conversation, that pepperoni was a kind of salami. And then recently, at a company event before ordering pizza and after a very confusing discussion of what toppings we order, it turned out pepperoni was actually a kind of a salami, but not everyone agreed. So by now I've learned that pepperoni is neither of them. It doesn't exist. It's listed on pizza menus, and when you order it, you'll get something for sure, but you won't know in advance what it would be.
• This isn't new, the realization was several years ago, but fits this list nicely: I thought, perfume was something for women. It turned out, there was perfume for men too.
• Parents used to teach, if you read in the dark (on paper, not on a screen, I must add), you're ruining your eyes. But if you think about it: wtf does low light do to your eyes? By that logic, you're constantly ruining your eyes while sleeping.
• For some reason I used to think, you could simply delete related entities bound by foreign key constraints in postgres, if you ran the query in a transaction. Once when I finally needed to do this, I learned the hard way I was wrong.

There's a lot more than this, probably I'll update this comment in the future. Or not.