LSystem.log has been showing one leaf_improvement4x4 error that I haven't been able to locate. It means that when you build road to square, the game has to always swap nodes (buildings of other improvement in that square). In this particular case the road would be placed but then it can't find suitable art files to place in the other nodes in that square. I don't entirely yet understand how this PlotSystem works but for 0.94 I did manage to locate one such bug - I'll have to try to find this one too. Could you tell on what era this happened? Since there's defined art files for each era...There was no unit in the tile. IIRC, it appeared when I built a road.