The event I will talk about is not mine, but instead of my maternal grandparents. I have never met my grandfather, he died before I was born so this was told to me by my mother. This happened when my mother was still a child.
My grandfather was working in a dispensary and sometimes came home at night and my grandmother used to wait for him. One night, she heard only one knock on the door and heard my grandfather's voice, saying, "Open, Mani, (that's my grandmother's name) it's me." She opened the door but there was no one out there, only felt a breeze that chilled her to the bone. Being weak-hearted, my grandma fainted.
Sometime later, my grandfather came and found my grandma lying on the floor. He woke her up and, at first, when my grandma saw him she was scared, but after being convinced that it was really my grandpa, she told him what happened. Finally, they came to the same conclusion--it was a ghost! My grandfather told her that henceforth, when he came, he would knock 3 times and only then should my grandma open the door.
I asked my mother the reason behind this and she told me that in the past, spirits liked to play that kind of prank but they only knocked one time and never more. My mother also added that at night we should avoid calling out people by their name too loud, as spirits passing by may "capture" the name and will come call you at night. Spooky, isn't it?
So from that day on, my grandpa always knocked three times and made sure never to call out to my grandma using her name.