I have never personally pranked anybody in a scary way, however my dad once done it to me and my brother.
Me and my brother were sat at the kitchen table, arguing and being just like children. Dad was at the counter, chopping vegetables and getting frustrated at our ramblings. All of a sudden he bellowed in anger and pain, "look what you've done you little sh**s!"
I ran over to the counter watching him clutch at his hand, and saw a full finger on the chopping board. I instantly went pale, honestly I was struck with shock. I started to freak out, and although the finger was fake in reality, my brain had created an image of a true finger with blood all over the place. Without the shock and my brain running away from itself, there was not actually any trace of red substance anywhere.
Within a few seconds his bouts of pain were muffled with laughter, and colour returned to my face as I realised what was going on. Yeah, I felt stupid...