I downloaded the "activity-lifecycle" demo project (http://developer.android.com/shareables/training/ActivityLifecycle.zip) and imported it into my eclipse ADT workspace. It built and ran perfectly. I started reading the code and found that there was suddenly an error (unexpected namespace prefix: "xmlns" found for tag RelativeLayout) in the activity_a.xml layout file for the line:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
There are two nearly identical files, activity_b.xml and activity_c.xml, in the project that have this exact line without generating a build error. I couldn't figure out how I'd managed to build and run it if there was an error, so I tinkered with copying the same code from the activity_b.xml file, but it also generated an error. Then I tried making the white space the same between the files, and suddenly the error disappeared. I made an insignificant change to one of the java files (calling Log.d()) and was able to re-build and run the project. Now, the next day, the XML error is back in the same place in the same file.
This is the kind of eclipse problem that seems to happen every other day and reduces my efficiency to about 10%.
Unexpected namespace prefix "xmlns" found for tag LinearLayout has work-arounds for this problem. Butt the problem is random and intermittent, hence a big time-waster for a developer who doesn't key in exactly the right search terms, so I'm gonna suggest that eclipse or Google fix it.