Steps to reproduce:
After login, I was able to select GNOME menu items from the menu panel, but their corresponding applications wouldn't come up.

I wanted to restart, but each time I'd click on System | Quit in the GNOME menu, nothing would happen. Also, another unusual symptom that was occurring during this time was that the (I don't know what you call it -- that box that you see when the music starts playing after you login) box wouldn't go away. It was stuck at the middle of the screen.

Finally, I looked at the system monitor and noticed a process called "at-spi-registerd". I thought it was a suspicious name, so I killed it. Believe it or not, everything started working again. As a matter of fact, all those failed menu selections began to be activated all at once. The quit screen opened 5 times or more (the number of times I attempted to restart the machine).

It seems like at-spi-registerd was blocking a queue (of menu selections that I'd made in the GNOME menu) from being processed. GNOME was queuing up all those menu selections, but not performing them immediately. Killing at-spi-registerd released the blockage.

I hope someone understands why this happened and how to fix it.

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Other information: