When you are doing regression testing with a release candidate, and you realize there's a bug in the old version that does not exist in the new one.
And you didn't know about the bug in the old release, and don't know how it got there. And therefore, you have no idea what happened to fix it in the new release.
@shatteredgears I think the right approach to such a problem is to forget about the fact that it's fixed in the new release and go about finding a minimal reproduction case in the old release, then see if you can come up with unit tests that catch it. At some point in that process it should become clear what fixed it between releases.
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