Along the lines of those radical fuel mileage figures.....
I have heard of people buying four or five identical hats of different sizes. Same trick, every couple of days switch your friends hat.
It can be quite entertaining watching you friend try to figure out why his head is a different size form one day to the next.
Another one; A guy at my workplace installed a water purifying system under the kitchen sink. When he the job was completed, there was a very slight leak. Knowing what a pain it is to work under the sink (removing everything under the sink, crawling around and working on you back, etc.) you can imagine how happy he was that it was finally repaired.
Of course those of us that were just watching the installation, got quite a laugh out of the three times he repeated the same job........ it might have had something to do with the fact that each time he would repair the leak and leave the room, we would just take a class of water and pour just enough on the hose fittings to make a nice little "leak trail!"
A little mixture (trail) of anti freeze and oil under you buddy's new car can be entertaining. Don't forget to splash a little on the oil pan..... much more convincing!