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1.
The act in which a girlfriend seduces her boyfriend for sex. However, once inside her, the boyfriend realizes that he has entered a dark and bloody mess. Often accompanied by a foul odor and a girlfriend shouting "Boo!" Most often occurring around Halloween.
Last night was Halloween and my girlfriend was on her period and I didnt know. To make a long story short, she gave me a Halloween Surprise. Needless to say, it was a mess! :(
by da hbomb October 27, 2011
 
2.
When you drill a hole in a bowl of candy and stick your penis in so the next dumbass kid who sticks his hand in gets a little extra "candy" than he bargained for.
"Tonight whoever comes to my door is gonna get a little 'Halloween Surprise'"
by marklar July 23, 2004