This is a great question. I would have to say that in every aspect of Psychology, there are going to be things that we learn along the way that stop us in our tracks so to speak. There are definitely going to be things that you would see in a different light than someone off the street with no knowledge of how the mind works on that deepened level. I think there would definitely be instances where you would see things either in a darker or brighter light depending on the topic based of the knowledge you would have as a clinical psychologist that would either help you understand or make you want to theorize and experiment to deeper understand. As for "messing up your head", I do not think that it would mess it up but I would definitely say you would walk away with a different perception on some things. It could either make you hardened to certain topics or make you more sensitive being as you would have a greater understanding of what ever the situation or mental illness would be. I will however from the medical standpoint say that you may run across that one case or that one patient that does stick with you as that is just the nature of the field.