Here is the stupid bit of navigation that I mentioned - spotted yesterday while in the car. Click the picture for a bigger version.
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Above is what Zumo 590 showed. Basically a T junction but with a filter for traffic coming from the right.
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Above is the Street view of the junction. The T junction is ahead and to the left. To the right is the filter for traffic coming in the opposite direction.
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Above: At the T junction. Intention is to turn right. The ovel shaped road in the estate opposite is where the Zumo was going to take me.
NB USA readers - we drive on the left over here in the UK.
I checked all of my settings. Nothing I had done could have caused this. So I thought a bit more. That filter road is for cars coming from the opposite direction. I cannot turn right into that road. If the maps have this junction as a no right turn, then the route suggested becomes a sensible one.
In other words, my best guess is that the 'No Right Turn' info in the map is slightly off.