Your project contains error(s), please fix them before running your application.

You get this error message, but there are no items flagged as errors in the project.  What is that?

The Android development version of rebooting, Project --> Clean doesn't resolve the issue.  What is it?

Go to:  Window --> Show View --> Problems

Embarrassingly, just found this one tonight.  Looking through the list, found the one problem for the app I was working on: 

error generating final archive:  Debug certificate expired 

What?  After a bit of Googling, turns out the debug certificate that Eclipse creates for you when you configure your initial Android SDK setup has an expiry of 365 days.  Happy anniversary!  

The fix:

1.  Navigate to the debug certificate and delete it or rename.  You can get the path to the debug certificate on your machine via Preferences - Android - Build - Default debug keystore.

2.  Clean your app again, then build it - should run without vague, useless error message this time.