You participate in a game show where you must choose between 3 doors. Behind 2 doors is a goat, but behind 1 door is a car. You pick a door, say door no. 1, but before you open the door, the host opens another door, say door no. 3, which has the car behind it. "Oops," says the host.
You lose.
I will direct to myself for the argument that switching doors when the host has no knowledge and just got lucky is meaningless. It's only smarter to switch doors if the host knowingly makes the correct choice.