I ran into this error a couple of times when trying to launch debug for an Android application in Eclipse. In all the cases I completely forgot what I had to do to fix this and this thread has consistently led me towards the solution.
However, not all the way. For some reason, doing a project Clean did not remove the jar file for one of the libraries in the workspace and that was causing an issue in another project that referenced the library.
In any case, for me the solution has been to simply search the workspace directory tree for *.jar, and remove the Jars for any of the projects that are about to be built (i.e. keep any 3rd party Jars for which no source code would be compiled.) So far so good..