JWT Melbourne, Australia has done this little ambient stunt for Vegemite - the stuff that like Marmite you'll either love or hate. Like most foods that have an acquired taste, Vegemite is really good for you, in fact it's rich in Vitamin B and a great source of natural energy. To highlight this JWT put these tricky stickers on cars around the Melbourne Cricket Ground, making it appear as if cricket balls had been shot straight off the field and into car windows. Who wants to bet the car owners got really pissed off when they first saw it?
Below is another car from another angle. If you check out the larger version of the image you can see the edges of the sticker.
Here's an interesting one, I don't know if JWT have film poster clients as well, else they might have hijacked this Jumper ad for their little stunt.