A guy at work has a turbo Subaru Liberty - he is fairly particular about it, keeps it very clean, so messing with his car is usually a bad idea. The guys had been throwing the term "Hommus" around a fair bit at each other - if you can't guess what that means, try
www.urbandictionary.com. Printed out "Hommus1", removed the number plate cover and inserted the piece of paper, replaced the cover.
We wanted him to see it before driving off - the joke would have backfired if he got done for number plate infringements on the road. So we put it on the back and dropped hints when he left for the day. Got a picture and sent it to a couple of others via e-mail.
Another guy at work threw half a cup of water at me in the face, then ran off. It was a hot day and being a black car, he put it in the shade and left the windows down a bit. I poured a full cup of water on his seat. Went back later and added more, but by the time he left after me, most of it dried.