According to the internet, it's difficult to simultaneously patting your head and rubbing your belly. Really? I can.
There's another one. "How smart is your right foot"? Sit down, lift your right foot and do clockwise circles. While doing that, draw number 6 in the air with your right hand. Apparently it's impossible, but not for me.
They talk of them like a brain glitch, that it cannot be done no matter how hard you try.