Our office has done the following over recent months. (1) A colleague who cycles in leaves his trousers in the office, and he came in one morning to find both trouser legs sewn up (we also nicked his saddle at one point). (2) Chocolate mousse left in shoes (same person alas). (3) (and stolen from The Office TV prog - but V V funny all the same) setting peoples office equipment in jelly. (4) Changing the autotext in Word (memorably, a collegaue sent out a few letters where we had changed "and" to "I like playing with my wang" - thankfully the letters were to a sister company. and (5), and my personal favourite, putting a colleagues phone receiver in your butt crack, taking a photo and then setting it as his wallpaper (have never seen a receiver changed so quickly in my life). All very juvenile, but great fun.