A Russian insurance company has created this novel ad in which a car is just driving safely along a road for 10 minutes. If you try to speed through the video though, the thumbnails show a head on collision with a huge lorry, instead of the peaceful journey.
The effect is achieved by adding in a frame from the car crash footage every second or so, at the right place.
We love things that demonstrate an idea in a simple and clear way, like this. If this is achieved, it's almost impossible for your message to be misunderstood and it's highly likely it'll be remembered—especially using something as common as YouTube in the delivery.