Er... your scenario (or something of that nature) is literally mentioned in the post: > Of the failures, 36 (62%) were tests not testing what they looked like

Er... your scenario (or something of that nature) is literally mentioned in the post: > Of the failures, 36 (62%) were tests not testing what they looked like they tested because of bad interactions with t.Parallel: the new semantics made the tests actually run correctly, and then the tests failed because they found actual latent bugs in the code under test. And if you want to keep your old bugs in under-tested programs, you can, that's why the new behaviour is opt-in.