Maybe test-cases should complete without BUG warnings?

If you run the unit tests with the --warn option, you fill find a lot of warnings. We deliberately test warning-paths, so that's not too bad...

... but you'll also see a lot of "Bug" warnings, and that's not so great. Some of them even cause stack traces.

I think we should go over these before 0.2.9 finishes, too make sure they aren't causing actual bugs.