A Blow Up Sheep.
My, er, friends (!) and I often play jokes on each other, and I'd started a new job only 3 days earlier. I was called to reception to take delivery of a parcel, and due to 9/11 security was tight and all unsolicited packages had to be opened at front desk.
So, I ripped open the box and out pops a fully inflated blow up sheep that was literally crammed in the box for maximum comedy effect on opening.......
I had a few funny looks for the remainder of my employment there, which obviously wasn't very long......