Seen on a beaten up old Ford Transit van in London owned by three 'rockers' in a house share, way back in the 70's :
'Don't laugh ... your daughter may be inside.'
Also saw this on a stockbrokers car outside his offices more recently :
'My other car is a Porsche.'
(It was stuck on a Lamborghini.)
Though the best sticker I ever saw was in the window of a house in Texas, shown in a BBC report on gun ownership -
'Is there life after death ... trespass here and find out.'
.