Some people may not believe what I am about to say, but it is completely true. There is an invisible golf cart at my old high school. It is yellow and always has a large trailer behind it, but you never see it until you have or are about to trip over it. It has been the cause of much trauma and confusion for me.
The first time I tripped over it was the worst. I tripped over the connector between the trailer and golf cart, giving myself a whopping black eye. It seemed to all happen in slow motion. As I was falling, the cart materialized out of thin air. A few weeks later, I smacked my shin in the same spot and in the same way. It happened once again a few weeks after that. Additionally, almost everyday, I would be taken by complete surprise when I saw the golf cart after sitting in the same spot for 20 minutes. I am not going crazy though because several of my friends had the exact same problem.
Moral of this story: reevaluate everything that you thought you knew.