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Some of a customer's polydatas end up with a single point and a triangle strip. Polydata with 1 point will pass the first test in vtkCompositeMapperHelper2::AppendOneBufferObject, which returns if poly->GetNumberOfPoints() is 0. A polydata can then squeeze through to vtkOpenGLIndexBufferObject::AppendStripIndexBuffer and underflow the calculation of triCount.

 vtkSmartPointer<vtkPolyData> poly = vtkSmartPointer<vtkPolyData>::New();
 vtkSmartPointer<vtkCellArray> triangleStrips = vtkSmartPointer<vtkCellArray>::New();
 vtkSmartPointer<vtkPoints> points = vtkSmartPointer<vtkPoints>::New();

 vtkSmartPointer<vtkTriangleStrip> strip = vtkSmartPointer<vtkTriangleStrip>::New();
 points->InsertNextPoint(0.0, 0.0, 0.0);
 strip->GetPointIds()->InsertId(0, 0);
 triangleStrips->InsertNextCell(strip);

 poly->SetPoints(points);
 poly->SetStrips(triangleStrips);

 vtkSmartPointer<vtkXMLPolyDataWriter> writer = vtkSmartPointer<vtkXMLPolyDataWriter>::New();
 writer->SetInputData(poly);
 writer->SetFileName("/your/path/to/bad_poly.vtp");
 writer->Write();

fyi: @chart3388