Somehow I two menus opened at once. I believe I had opened the backpack menu, pressed escape and continued playing. At some point I went to change class and both menus got stuck open at the same time and refused to close. Clicking made my character fire his weapon and as I mashed at my keyboard in frustration other menus opened but I was unable to close any of them.
I had to kill the application as although the game was running fine the menus were unresponsive.

System info http://pastebin.com/TvJeVdcg