Dear Maintainer,
When adding keyboard shortcuts that are incremental (alt+shift to change language; alt+shift+tab to reverse window switch), the shortcuts will clash. Because alt+shift changes language, alt+shift+tab will do nothing. This wasn't always the case (I had it set up like this in Debian 5.0). This bug was fixed in Ubuntu a couple of years ago; I added the lines from the Ubuntu patch to the 2:1.12.4-6 version of xserver (no clue if this is the correct way of doing it).
To replicate the bug:
Add alt+shift as a switch language shortcut.
Try to use reverse alt+tab (if not configured, configure it in dconf).
alt+tab stops working.