Last Thursday I noticed for the first time that when you drag a map in Google Maps, the scale in the lower-left corner changes, though the zoom level is the same. That's perfectly natural with the Mercator projection, but still comes as a surprise when you see it for the first time – after all, the static paper maps don't do that. Moreover, if you zoom out to the furthest zoom level you still see the scale, but it doesn't make sense! You obviously can't apply it to the whole map if you don't want Greenland to be bigger than North America.
Another funny thing was a question for the Russians who learn English: how do you translate the names of the rainbow colors? It has stumped everyone so far: we all know the seven rainbow colors in Russian, but the colors #5 and 6 (голубой and синий) are both usually translated as blue. I even thought that maybe in English-speaking countries the rainbow is quantized into 6 colors, but wikipedia clarified that these two are actually blue and indigo. It feels quite stupid: you definitely don't know the language if you can't even name the rainbow colors.