Steps:

1. In a plugin or the console, do:

other_window.focus_view(other_window.views()[0])

2. other_window will be apparently focused (the current view will be defocused), but it will not be in front, and for the user it appears that nothing has happened.
3. Alt-tab to another application.
4. Alt-tab back to Sublime Text.
5. oher_window will now be in front, and will have focus.

This bug prevents me from writing a particular plugin.