I went to Germany for my honeymoon. All of their symbols on road signs were easy to understand except WC. I had to ask about that one. I'm not sure why the Vienna Convention on Road Signs and Signals didn't just use a pictogram of a toilet for that sign.
Some British main road junctions bear yellow signs with black squares or triangles. Often wondered what they mean or how you use them - I think they are meant to suggest alternative routes if the main one is closed or congested, but I am not sure how. Perhaps they are explained in the Highway Code I ought read again now and then!
Also small green signs carrying only three letters: "RVP" - but I think they refer to certain emergency-services functions, and are not for the ordinary motorist.
I was for a time puzzled by a green "traffic light" I noticed only after dark, wondering what road it was on, until one day I drove past in daylight and realised it was a signal on the nearby railway! I think it had been installed not long before I first saw it.