Various smoke tests fail when built with --js-engine=v8-3.25. I haven't poked into every single one, but the ones I have looked at are all some variation of:
2014-07-15T05:19:26.528+0100 Error: DBRef needs 2 arguments
at Error (native)
at /home/mwhudson/mongo/jstests/core/shelltypes.js:14:5 at /home/mwhudson/mongo/jstests/core/shelltypes.js:14
failed to load: /home/mwhudson/mongo/jstests/core/shelltypes.js
so I suspect that they are all or at least mostly symptoms of the same underlying problem.