Keyboard shortcuts randomly stops working, it appears to primarily affect shortcuts involving the Alt-key.
I have seen this issue in 12.04 as well. I'm now trying to research the issue in more detail, so I'm hoping there is some way to log what happens when a shortcut key combination is pressed.
I'm still very interested in knowing how xubuntu connects a shortcut with its intended action, but I did something that may have solved the issue:
Going through dconf-editor under
I removed the the shortcut "Alt"
The issue hasen't shown up since.
This indicates that the original issue may be triggered by compiz grabbing the alt-key sometimes.