Re: TDD still a thing?
Yes, it could be caught by a very simple unit test. The problem, as always, is combinatorial explosion: There are a huge number of potential problems that could be caught by a similarly huge number of trivial unit tests. Someone or something1 has to create those unit tests.
Per a comment above, there was a unit test which exercised the code path in question, but its verification logic didn't check all aspects of the output, and so missed this error.
1There's a fair bit of interesting research into automating test creation.
Biting the hand that feeds IT
