You know how it is. You find yourself playing a game, getting slowly sucked into it, when all of a sudden there is a door barring your way that won't open up no matter what you do, or a ruin that blocks your way in all directions, or all of a sudden not being able to jump over a small 1 foot high structure and BANG you're all of a sudden very aware that you are playing a video game. What would be your biggest immersion killer?