Alien. I saw it when I was too young to see it, and my older sister traumatized me by telling Aliens were real and ate bad little brothers. As a punishment, my nightmares got so bad at one point my parents made my sister wake up and go to my room to calm me down. I still have problems watching that movie. I have no problems watching the rest of the movies in the series (except that one that sucked, really, really bad, when the Alien thought Ripley was its mom, that was horrible) Also, I think my fear of snakes is related to the first alien movie. How fast they strike, fangs/extra teeth: there is a relation there.