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.
No, I was actually wrong, sadly. It does matter if it's random or intentional. It get's pretty hard to grasp, but there are ways to prove it.
In that example, if the choice is random, and he doesn't reveal the car...he had a 50% chance of the 66% chance to reveal the car, and didn't...meaning the remaining door is also 33% chance, just like the door you already chose. Or, since they're equal chance, now each is 50%, and it doesn't matter if you switch.
If he intentionally removes a known non-car, it is your example; 33% vs. 66%, and you should switch.
It can be a bit much to wrap ours heads around, since the whole concept of predetermined but random outcomes isn't really standard fare, but it does make sense.
Where it gets crazy is, to make your decision on whether you should switch doors, you need to know how the host made his decision in eliminating doors. It gets pretty mindfucky.