"Warning: This warning label may be harmful or fatal if swallowed".
"Warning: This bag is not a toy"
"Caution: The beverage you're about to enjoy is extremely hot"
A semi-related favorite I've personally seen was in Steamboat Springs, Colorado where a bridge crossed one of several deep gorges. On the side of the road, just before a precipitous drop-off was a sign reading "no throwing of objects from bridge". Looking way down into the gorge were about 20 of those signs at the bottom.
