This has happened to me with two different people. Both times my face apparently transformed into the face of the person's father - while we were sleeping together. Only one of the times was after the person smoked pot. It was scary to say the least, and scarier when the first one ran to the kitchen and grabbed a chef's knife.
Life lesson: before you get in a relationship, ask about any history of mental disorder or medications being taken. Some people don't tell you about these things until they happen.