Playing Super Mario 64 for the first time, on a Toys R Us demo unit, before the console was out.
I remember running around in 3D and being amazed by how responsive the controls were. How easy it was to navigate that 3D space. And then I saw the fence around the moat of the castle and thought, there's going to be an invisible wall. If I try to jump over it, there will be a barrier and I'll just bounce back. And then, when I jumped over the fence and landed in the water, that was it. The most, naturally, blown away I had ever been because of a video game.