The act of sneaking up on someone and slapping them with your penis hard enough so that it leaves a "mushroom" type bruise.
God damn that mushroom surprise hurt.
When you dip your dick in ink, and then proceed to smack it on a girls face while she's sleeping. She'll see the imprint in the morning, and be surprised by the mushroom stamp.
Yo dude, my girl woke up this morning with a mushroom surprise.