ingrid added the comment:

This seems to be caused by a bug in TurtleScreen.update/turtle._drawturtle. 
When the speed is set to zero, the tracer method is used to regulate drawing 
circles, and when called with a positive integer, tracer calls update. Update 
iterates over the list of existing turtle objects and then updates them and 
calls _drawturtle on them, then redraws them, but doesn't draw lines or stamps 
so they'll get drawn over by shapes and images. I attached a snippet that shows 
the bug happening when you call update directly, if you switch the last line 
and use tracer instead the same thing happens. I think the way to fix it is to 
add stamps and lines to _drawturtle so I'm going to work on a patch that uses 
that approach.

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nosy: +ingrid
Added file: http://bugs.python.org/file36646/update_example.py

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<http://bugs.python.org/issue21077>
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