You're in an app that is showing a dialog (for example "Do you want to save....") or a modal window (for example a settings window).
In app switcher mode, alt-tab to another app.
Then alt-tab back to the app with the dialog or modal window.
Then the main window of that app is shown and you have to alt-~ to get the dialog or modal window back.
Is it possible to restore the active window of the app you're tabbing back to?