Sometimes, a funny sticker on the rear of the truck is a laugh, but often coupled with a message.
Here are some examples:
During the 80th, a sticker was hip in Germany, what said: "Ich bremse auch für Tiere". ("I'll brake for animals, too.") Now, I'd seen one with the words: "I'll brake for animals, catholics and muslims, too, but NEVER for stupid car drivers!"
Big letters: "Your distance is ok." Smaller letters: "Keep more distance!" Very small letters: "If You can read this, You are already in hell!"
"Dear cops! Begging and hawking forbidden!"
"Dear car driver! I'm only rolling in front of You, to produce jam's and to annoy You."
"You are angry about the truck in front of You? Well, roll home Your cheese by yourself from Holland!"
"If You prommise me, to eat roted food, to read old newspapers and to spend Your car no spare parts more, You'll never see me again!"
"Of course, I'm slowly! I wanna see You running with a 25 tons coil on Your back!"
Well known are stickers like: "No staying in the dangerous zone / surrounding of this crane!" (or alike that). I have a t-shirt: "No forklift staying in the dangerous zone of the truck driver!" ("Der Aufenthalt des Staplers im Gefahrenbereich des LKW-Fahrers ist verboten!")
If someone knows more such sayings, please write down.