Mark French, a professor at Purdue, created a cannon that can fire ping pong balls at 900 MPH. The serve is certainly not returnable and it leaves a hole in the paddle, even though ping pong balls are so light.
As Gizmodo points out, the ball has an equal energy and momentum of a bullet, so it could potentially be very, very dangerous.
The ping pong ball gets fired at about 5:40:
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