Comment 7 for bug 641335

@mdz Please do try with very latest unity, mutter and appmenu, as the fixes didrocks mentioned did touch precisely the code where your seeing the crash. Otherwise, if your running on an intel machine, starting unity from VT1 like this would give us a chance of a backtrace:

export DISPLAY=:0
export UBUNTU_MENUPROXY=libappmenu.so
gdb --args mutter --replace --mutter-plugins=libunity-mutter
(gdb) r <enter>

Then, switch to VT7 and, when it crashes, the screen should freeze and you can switch back to VT1 and hopefully get a trace. It's a pain to copy gdb traces off a VT, though, so having a camera handy is useful :)