Like I said: yes, the engine failed, and the engine shouldn't fail. But it's unreasonable to expect that if only the engine had continued working, the test flight would have gone fine.

This is not a product that is marketed as something still in testing. Rails is supposedly mature by now.

A closer analogy would be if you were an airline and a technician from Boeing made some Boeing-recommended safety changes on a plane before it took off and then the engine blew up.

Sure, you should have had proper oversight of what said technician was doing but the bigger part of the blame is on Boeing for a faulty safety change.

I'm not talking about Rails, I'm talking about Github's mail-sending program. Rails is the engine. The email sending script is the experimental aircraft.

Blame Rails, I'm fine with that. But it's not going to prevent a serious problem with the email sending script in the future. To do that, you need to write some tests.