Growing up, every Sunday I'd drive w/my mother to visit my grandparents. On our drive there, we would see a tank along side of the road that had this sign on it:
NO DRINKING
WATER IS NOT POTABLE
Now it makes perfect sense to me. But as an elementary-aged child I was so confused about the meaning of this sign. I would sit in silence just trying to figure it out. I was reading it in my head as a sentence "No drinking water is not potable". The (apparent) double negative confused me. Was this sign saying that drinking water is always potable? Or something else? If drinking water WAS always potable, why did there need to be a sign to inform people of this? I seriously twisted this around in my head week after week for a long time.
I finally one day asked my mother for some clarification and we had a good laugh. It was as simple as "Don't drink the water!".