[Isomorphic]

Well don't leave us in suspense! What's the ClassCastException? (Full error message and stack trace).

[smcdonald]

I'm very much a novice with web GUI development. Like I mentioned in my previous post, I can't figure out how to get at the full stacktrace.

[Isomorphic]

You should already be seeing more information than just the classname of the Exception, without taking any special action. Possibly you have code that installs an UncaughtExceptionHandler (see core GWT docs for GWT.setUncaughtExceptionHandler()) that logs way to little information.