Not only that. Often mitigations can be placed even if the actual bug cannot be reproduced. Like many others in the thread suggested.

I've encountered several impossible to reproduced bugs in the past. And what I (or my team) would then do, is re-architecture (refactor) some pieces of software so that we could reproduce it. Like e.g. better logging, specialized layers/adapters/services, simpler logic, and -above all- better testability.

If it was me running the project there's enough information in that thread to piece together an exploratory testing plan around the issue that might allow us to isolate it, and I'd set aside some time for the team to do that.

Whilst obviously not lethal, this Thunderbird bug sort of reminds me of the Therac-25 incidents in the 1980s. Very occasionally the machine would give patients massive overdoses of radiation. This bug wasn't easy to reproduce (thankfully) and turned out to be due to a race condition.

But of course, you can't find a problem if you don't investigate, and if it's a serious problem that's been documented then, as engineers, we can't just hide behind non-reproducibility as if it's some sort of magic shield. We have a responsibility to investigate and isolate the problem ourselves. If we don't do that we are effectively washing our hands of our own creations.