We report a novel mode of quasi-static oscillatory crack propagation when a cutting tool is driven through a thin polymer film [6,7]. In our experiments, for tools with a large enough width, the crack tip follows a highly periodic path, leaving behind a striking oscillatory pattern. Taking into account the separation of the film's bending and stretching energies and using classical fracture theory, we propose a geometrical model that accurately reproduces the behavior observed experimentally. Numerical integration of our geometrical rules accurately reproduces both the shape of the fracture pattern and the detailed time evolution of the propagation of the crack tip [16].