[ubuntu] Ubuntu 13.04: Cannot shutdown if more than one users lock their screen

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http://ubuntuforums.org – On Ubuntu 13.04 raring, I've noticed a bug when one or more users lock their screen. In lightdm, clicking on the shutdown causes it to freeze, which is recoverable by entering/exiting tty. This is annoying especially when the users on my computer switch accounts rather than logging off. Is there a way to fix this bug? Thanks. (Hardware)