Oh, sometimes upon loading the map up in a new round, a graphics glitch appears. It's nothing big. I have found a way to deal with it rather than restarting your game. When you see it, just hit cntrl+alt+del, go to task manager and once you hit task manager and it trys to take you back to the manager, it fixes the game right up. When you close the game your task manager will be open but its better than having to restart the game. Hope this helps, has been working for me anyway.
EDIT: Don't ask me why or how this works I only know that it does.