Linux (Arch) intel amd64 + NVidia 1070M
Short description of error
2-way symmetrical mesh (x and y) could not be properly loop cut when Topology Mirror was enabled. Some midpoint vertices attempted to mirror to the other side of the mesh, causing massive deformation of shape.
Exact steps for others to reproduce the error
Attempt to loop-cut-and-slide the car in half (dividing the longest edges). Mirror modifier is not necessary to reproduce. Only disabling "Topology Mirror" on the tool drawer will allow loop cuts to behave properly. Many other harder-to-describe loop cuts fail to meet expectations (on this same model).