I didn't learn this today. But if you will indulge me...
Years ago, when I was about 7 or 8, my cousin got a chess set for Christmas. We started to play and I quickly picked up the moves and though I found it complicated I found it interesting.
Now, my cousin was super into sports etc. He was very competitive. But he was a terrible loser. He always had to win.
This put me at a disadvantage since I had to learn chess from my opponent. One day I found myself in check against him. I looked at my admittedly poor position in the game. Ran through all the moves and counter moves I could think of, then realised "oh I can just take the piece with the king, problem solved." So I did.
My cousin obviously irritated that he hadn't predicted the move told me that when you are in check, you can't take the piece that put you in check. So I believed him.
Years and years, and many chess games later, I was playing chess on an app. Using the "knowledge" my cousin gave me, I thought I had the computer cornered. Quite an event for me! Until I saw the computer make it's move and take the piece that put it in check!
I was stunned! My cousin had lied to me and for 30 odd years, I had been playing with one arm tied behind my back. I learned you definitely can perform such a move!
The nasty little cheat!