I once had an experience when I was dreaming that has burned itself into my memory for years. It was during a regular dream, I was walking beside a road during the fall and I was alone, there wasn't anyone or anything around me (one of my tamer dreams by far). I was just walking, then it's like something hit me mentally. I looked around and I knew something was wrong, I didn't know what or why, but everything was... wrong. I tried to reason it out, but it felt like my brain wasn't working right and the harder I tried to think the less things made sense. Eventually the dream aborted itself and I woke up.