shm sometimes tries to extrude patch edges that have no corresponding mesh edge

We're seeing problems (very occasionally) when adding layers (through a faceZone)

[25] --> FOAM FATAL ERROR: [25] Problem: cannot find patch edge 156096 with mesh vertices (942326 942924) at ((0.818753115788 -0.340911030206 0.660034044557) (0.818405891096 -0.340296224432 0.66104959179)) is not in face 3658538 with mesh vertices 4(943717 942326 943719 942924) [25] [25] From function static void Foam::addPatchCellLayer::setFaceProps(const Foam::polyMesh&, const indirectPrimitivePatch&, Foam::label, Foam::label, Foam::label&, Foam::label&, bool&, Foam::label&) [25] in file polyTopoChange/polyTopoChange/addPatchCellLayer.C at line 498.

It does not find the correspondence from patch edge (derived from patch faces) to edge on a mesh face. Instead it seems to correspond to the face diagonal.