Another obscure error that I got while doing while following the Google App Engine tutorial was solved in about the same way, changing the JRE used by the application.
I got a pretty mysterious error message, "Expecting a stackmap frame at branch target 39 in method" (etc etc ...) while running a simple servlet that was just trying to access the users functionality of the google appengine API.
In my case (I am working on a Windows XP box, developing with Eclipse Indigo) I found out that I had this error when using JRE 7. No issue when I use JRE 6. When using JDK (both 6 and 7) still no issue.
See Window, Preferences, Java, Installed JREs, to check which JRE are seen by Eclipse, and which one is currently used for your application.