My earliest memory of having a bad nightmare is from when I was about 11.

I had gone away on school camp and was sleeping in a stupid tent. I struggled to fall asleep but whe I did I found I was at home in my bed room. I heard music playing outside so I ran to the window to have a look. In the street there were a number of clowns on unicycles riding up and down while circus music played in the background. I shouted for my mum to come and see. She came in slowly through the door and looked at what I had found. She was quiet and unsurprised by the scene then she turned towards me with her mouth open and turned in to Penny Wise the clown from the movie IT. She had sharp teeth like daggers and as she advanced I screamed and woke up with my head half way out of the tent. 

It still spooks me to this day.