I assigned this to myself when I thought this was going to be an easy fix (since I knew encodings._cache wasn't getting cleared), but it looks substantially more complicated than that.

I checked the codec entry was getting cleared by tearDown() correctly, and explicitly clearing the cycle between the test case and the codec implementation didn't help anything.

Restoring the cache to its original state did help, but only a little bit.

It may be one of the other test cases that's the problem, or else there may be something genuinely odd going on with the exception chaining and local variables.