Some user-made models for Quake have inverted lighting normals. This makes directional shading go wrong, specially in features such as directional lighting from dynamic lights.
Skiffy's Shambler model has an inverted Y normal, and Capnbubs' models have inverted X and Y normals.
Opening the models in qME, scaling the models by -1 and back to 1 makes qME recalculate the lighting normals, but the lighting smoothness across non-continuous edges is lost.