At the end of the alpha, players found a way to exploit the game. Game didn't allow placing claims next to each other, to avoid performance issues. Players found a way how do it and built a city.
Also, original voxel engine had smallest voxel unit size of approximately 15x15 cm cube. Players found a way how to circumvent this and created so called micro-voxel which can be much slower, allowing them to build furniture, weapons and stuff. Luckily, engine is robust enough to run without issues even with large number of micro-voxel. However due to alpha state of the game and lack of optimization, player had serious performance issues in the city they build that's mainly because there's memory leak/cache problem and because there were so many of them in city at the same time (over 200 players).
Also some cool buildings in this video: