Unexpected…

flat_on_flat.geo
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			nb_points = 11;
			nb_points_height = 11;
			height = 2.0;
			length = 1.0;

			Point(1) = {-length/2, -height/2, 0, 1};
			Point(2) = {length/2, -height/2, 0, 1};
			Point(3) = {length/2, 0, 0, 1};
			Point(4) = {length/2, height/2, 0, 1};
			Point(5) = {-length/2, height/2, 0, 1};
			Point(6) = {-length/2, 0, 0, 1};
			Point(7) = {length/2, 0, 0, 1};
			Point(8) = {-length/2, 0, 0, 1};

			Line(1) = {1, 2};
			Line(2) = {2, 3};
			Line(3) = {7, 4};
			Line(4) = {4, 5};
			Line(5) = {5, 6};
			Line(6) = {6, 7};
			Line(7) = {8, 1};
			Line(8) = {3, 8};

			Line Loop(1) = {1, 2, 8, 7};
			Line Loop(2) = {3, 4, 5, 6};

			Plane Surface(1) = {1};
			Plane Surface(2) = {2};

			Physical Line ("loading") = {4};
			Physical Line ("fixed") = {1};

			Physical Line ("sides") = {2, 7, 3, 5};

			Physical Line ("contact") = {6, 8};
			Physical Surface("lower") = {1};
			Physical Surface("upper") = {2};

			Transfinite Line {6,4} = 5 Using Progression 1;
			Transfinite Line {3,5} = nb_points_height Using Progression 1;

			Transfinite Line {1, 8} = nb_points Using Progression 1;
			Transfinite Line {7, 2} = nb_points_height Using Progression 1;

			Transfinite Surface "*";
			Recombine Surface "*";