On turn 1257 after exploring the map (I was only exploring, I had neither built nor researched anything), I decided to build a city near a crystal. The game hung and didn't seem to process that I had clicked the build city b utton, so I clicked it several more times. Gving up on building the city, I ended the turn. At the start of the next turn, about 6 city building pop-up messages came up, and I confirmed them all. Several stacked on two tiles, and one was built alone on a tile. Cities stacked on the tile where my soveriegn had been, but my soveriegn had never touched the other two tiles where the cities were built, this seems to be random (or my sovereign had moved and I hadn't seen it). Six cities were built over three tiles, my population rose by 60, so six cities were definitely built. The link is a screenshot of the stacked cities, there was a graphical glitch effect in the borders of the cities similar to graphical glitches that occur in 3D animation software when two textured polygons are placed in the exact same space and doesn't know which one to display.