I learned this from a coworker who scared his teenage daughter with it. This is a simple prank that can be done quickly and anywhere you can find toilet paper (or paper towels). It's simply drawing a spider on the toilet paper. I just searched Google images for the type of spider that I wanted to draw. I chose to draw a black widow. I grabbed my black and red pen and headed for the commode.
One thing to keep in mind is where on the roll to draw the spider. If the toilet paper holder is located somewhere that is not initially seen when entering the bathroom, you could probably draw the spider directly on top so that the victim sees the spider as they are reaching for some TP. Otherwise, you may consider unrolling a little (or a lot), drawing the spider, and then rolling it back up. It could be like Spider TP Russian Roulette - you never know who's going to get the spider [insert evil laugh].
Now get out there and scare the poo out of people and remember to vote in the Pranks Contest.