if application "unmap" window, but not destroy. WalkWindowStack still try focus the hidden window and alt-tab switching act strangely and miss some other windows.
this bug triggered by retrovol (tray icon gtk2 alsa mixer program).
(i use archlinux)
if retrovol's "Full Window" (in tray menu) window open and than closed, retrovol not destroy this window but only hide(unmap). after this bug start.