If I look at the v4 source for popBackStackImmediate it immediately fails...Depending on what action you were doing that wasn't allowed, this might ...
Navigation Tabs with the ActionBar TabHost Overview Walkthrough - Creating ...Part 4 - Providing Backwards Compatibility with the Android Support Package ...
// sets current tab by index to the first tab on the bar getSupportActionBar().setSelectedNavigationItem(0); We can also access arbitrary tabs, check...