You pick up a chick in a bar and you are so horny that you decide to bang her in the field behind the bad. That is where the surprise comes in to play. While pounding her, you don't notice that your swinging ball sack is hitting the ground, which happens to be covered in pine needles. Ouch....doesn't even cover it.
"I was boning this bitch behind O'Tooles and ended up with a Pin Cushion Surprise. I'm still picking pine needles off my ball sack."