Q:

precompiled header name gets stuck

I'm new to xcode and I must be missing something.
Create a new command-line project.  Call it tempprog (for instance).
Select Project/Edit Project Settings.
Edit the name of the prefix header (tempprog_Prefixz.pch).
Build - it will fail, of course, looking for tempprog_Prefixz.pch
Now change the name of the prefix header back to tempprog_Prefix.pch.
Build again - it should work, but doesn't.  It's still looking for tempprog_Prefixz.pch  
Cleaning does not rectify the situation.  What am I missing here?  Where is the Prefixz.pch name being retained?  If you look in the build output at the ProcessPCH command you can see that it still wants to use Prefixz.

A:

It turns out there's a system of setting priorities.  
From Working With Xcode Build Settings:
The levels at which build settings can be set, and their precedences are:
xcodebuild command-line flags (only applicable for command-line builds)
The target, editable via a target's Info inspector
The project, editable via a project's Info inspector
Xcode application settings, as set in Xcode > Preferences...
Xcode's built in defaults
The user environment, perhaps as set in a user's ~/.MacOSX/environment.plist file

