[grandpoobah]

"The said we had a new client, they had a weird name."

This is the material piece of information which lead to the fix. Your TDD simply made it faster to locate the offending line.

Seems like a really poor justification for TDD imo.

[dns_snek]

I agree, this sounds like a completely normal process of finding and fixing bugs.

The stack trace tells you where the error is occurring, your intuition leads you to the relevant caller function that you should test, stack trace or logs should lead you to the offending parameters (i.e. the weird name).

With these three pieces of information you can write a failing test and implement a patch.

I don't consider this approach to be "TDD", just "software development with good test coverage".