Someone opens a cupboard door and ball bearings fall out and fill the room entirely up to their neck.
How it's achieved:
The cupboard has to be one of those ones that is attached to the ceiling and high up, like you get in a kitchen. A hole is made in the ceiling above the cupboard but when the doors are closed you can't tell. The room above the cupboard has a tank filled with enough ball bearings which would then feed thru the hole and rush out of the cupboard doors once they get opened. The doors can be stuck shut or heavy to avoid the balls falling out on their own without the doors being touched.
Surely, this would create utter shock for the vitcim. I came up with it by taking a basic prank (stacking plates so they fall out of a cupbaord when opened) and taking it to it's most extreme level.