The in-browser interpreter and the server-side interpreter used for checking do not agree.
Long-term, the "environments" idea used to select what environment is used for server-side checking will also need to affect what Python version/features are available in skulpt within the browser. This could be a major issue, since "upgrading" skulpt could be a major effort. Still, we'd probably need to support both the current version of skulpt and a python 3-based version, since the media computation/Jython-based checking should use the existing skulpt version rather than the new one.
Maybe it isn't too bad--maybe there are only two versions to consider (Jython @ v.25, Python native @ v3.3?).