Faux spilled coffee is an easy, realistic DIY prop designed to inspire panic and induce profanity.
Just look around. You're probably surrounded by irresistible, deserving targets: The fussy geek-of-a-roommate and his do-not-touch-my PC... that annoying little sister (who wouldn't loan you 2 bucks last week) and her homework. Then there's mom, who forces you to clean the whole neighborhood, (but really just your room) and her "Use a coaster, honey!" antique table.
You can literally punk anybody in your immediate vicinity... and here's how:
As any great prankster knows, properly staging this tomfoolery will produce optimal, "WTF!" results. It's preferable that you be glimpsed with the cup in your hand, prior to the punking. Then again, maybe not... if you want the victim to blame someone else. That's what I call a 2 for 1 prank! >;-D
This spilled coffee cup can also double as a seat-saver. Discreetly place on the tabletop in front of the seat or directly on the seat you'd like to save. Offending interlopers will give it a wide berth.
You may also opt for The Sloppy Spoon Prank (sans coffee cup) as pictured below. It's an equally effective prop that can be carried in your pocket, purse or glovebox. Don't leave home without it! ;-)
Caveat: Due to the unpredictable nature of human excitabilty, this prank may not be appropriate in the presence of priests, pets, grandmas or children with sensitive ears.