ALT+TAB loses focus

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http://askubuntu.com – I often use ALT+TAB but sometimes, not always, after 0.3 secs it loses the focus on the newly switched window. No arrow on the right of the icon in the launcher, no name of the app in the top-left corner, no cursor active, but I see the correct window and the previous one is in the background. This happens with any window, often I switch between terminal and Aptana (so I press ALT + TAB), and there's no shortcut assigned to ALT itself. Anyone has any idea what could be wrong? Thanks (HowTos)