One of the biggest causes of accidents in Germany is when people change lanes without really looking. Volkwagen Commercial Vehicles invented "Side Assist.“ showing the dangers in the driver’s blind spot via signals in the wing mirrors. This way, with just one look,you can spot any danger behind or alongside the car. To create awareness about VW's side assist, they inserted a card into the housing of wing mirrors on all competing commercial vehicle brand. The copy is only half visible and only fully legible when viewed in conjunction with the mirror, letting the drivers see dangers they would not otherwise recognize.
The translation of this ad is basically: Seeing only half of it is not enough.