This happened to my family when I was about 1 or 2 years old; I am now 15. My mother and I were living with her mother who lived in an old house with a troubling history. Around the late 1950's a man went insane and murdered his family which consisted of a wife, 2 sons and and a young daughter. Their bodies were found in a small shut-off cellar that my grandma has in her basement.
My mother and I left for a doctor's visit, but my mother didn't tell my grandma that we were leaving. My grandma was doing laundry in the basement when we left and on her way up the stairs she saw a little girl around my age so she assumed it was me. 'I' was sitting on the top stair and asked my grandma for a glass of water. My grandma got the water and was walking back towards the basement door to give 'me' the cup when the doorbell rang. My grandma went to answer the door and as she did, she put the water on the dining room table. It was my mom and me at the door, my mom had forgotten something so we had to go back. Startled, my grandma asked how could my mother had me the entire time when I was just in the basement with her. Mom confirmed that I was with her and that I hadn't been at home. My grandma told her what had happened and when she went to show my mother where the girl was sitting, the glass of water my grandma left on the table was where the girl was sitting. The water was gone from the cup... creepy.