Automatic extra perimeters
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By default, Slic3r increases the number of perimeters in those layers whose profile is sloping. This logic avoids gaps. However it can be disabled manually using the Disable extra perimeters option.
Michael Andresen (@blddk) posted a nice example: http://captain-slow.dk/2012/06/16/slic3r-0-8-3/
More details can be found at https://github.com/alexrj/Slic3r/issues/198