I relatively often meet 4 of these:

• no speed limit (use the normal limit for this type of road)

• car tires may defy laws of nature (slippery road, usually followed by a sign saying it applies during rain)

• speed camera ahead

• no water polluting goods (not very common, but occasionally comes up. There is also no dangerous materials with an orange trapezoid instead of an ellipse)

I also saw don't drive off the pier (around ferries), watch for skiers (in the mountains with cross-country skiing routes), and warning about planes, although in different design (around airports).