My girlfriend, Heather, LOVES the "I'm pregnant." pranks. Has gotten me a good number of times over the past 12 years. Figured it was time to get her back.
Around Halloween I buy the bottles of fake blood. Last winter I poured a good puddle of the fake blood in the kitchen floor then laid face down in it when I saw Heather, my girlfriend, pull in the driveway. She came thru the door, stopped, then I heard her make this weird sound like a cat, then practically tear the door off the hinges as she is running outside. Luckily I made it outside before she could complete the 911 call.
She stayed at her sisters house for a few nights before coming home. All is well now, and we always share a good laugh over it when it gets brought up, but she never fails to remind me that paybacks are hell!