In a new game, I built 2 settlements, successfully linking them with a road. Once I built the third settlement, I tried to order both original cities to build roads to it, but I believe I accidentally clicked cancel both times, instead of accept. When I went back after a couple of turns and told city 2 to build a road to the new city (3), clicking 'accept' caused the game to hang. After ~30 seconds, I CTRL+ALT+DEL'd my way out of there and killed the process.

debug.err = http://pastebin.com/f3db2624e