I remember making the exact same mistake (accidentally using a single function call in a schema) back in 2010. No LLMs required.
The bigger culprit is probably a lack of testing / debugging. This error would immediately get caught if you simply registered twice on a test instance.
Testing and debugging isn't just a matter of stepping through code, it's an exercise of seeing where your mental model of the codebase is faulty versus the current reality of it.
I've encountered similar problems and they'd be fixed in a matter of hours, not days.
I wonder why this victim didn't ask ChatGPT to identify and fix the problem...
Because if humans had written and reviewed the code, multiple team members would have had to have learned Python, and SQLAlchemy specifically, which, even if the mistake was initially made as many times as ChatGPT did, there would have been multiple independent opportunities for it to be caught and questioned and the relevant knowledge shared during development.
ChatGPT may be able to crank out immense volumes of superficially functional code, but if its your only “teammate” that understands the libraries used and touches the code, its a huge single point of failure.
So /that's/ where ChatGPT learned it! :)
Friendly reminder: check if your codebase is actually testing this!
One of the interesting consequence of running unit tests with a fresh database everytime is that problems related to unique constraints seldom get caught by unit tests.