I've been preparing a map of a relatively small area of Montreal, Canada for a manuscript I am writing. This is, of course, about some snails I collected back in August when I was there. As my base map, I downloaded a suitably scaled map from Google Maps and then started adding my own layers to it using Photoshop. When I noticed some discrepancies between my field notes, how I remembered the roads were and the Google map, I went back and compared the map with the actual image in Google Earth. That's when I noticed this peculiarity of Google Maps.
Here is a portion of the map. It shows that the Autoroute Décarie crosses over the 4 diagonal roads, Rue Sherbrooke, Boulevard de Maisonneuve, Chemin Upper Lachine and Rue Saint Jacques and the railroad. Right?
Wrong. Note what happens when I increase the scale: those roads, including the railroad, suddenly shift up and cross over the autoroute.
That's the correct spatial relation of all of those roads with respect to each other, as you can see in the Google Earth image. There is still another error, though. Chemin Upper Lachine is supposed to under, not over, the railroad. Even at the highest scale, Google Maps doesn't fix that error.
This isn't some quirky behavior that happens to be specific to the map of Montreal. I have noticed it happen even in Maryland. I don't know what the underlying rationale is and if this is a standard practice in cartography. In any case, this could create one heck of a confusion if one is driving in an unfamiliar area using a map printed out from Google Maps. It happened to me once.