Welcome to your interview.
The question is on your right: Debugging Iterable API 400 Errors. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
How do you troubleshoot a client's API call that is returning a 400-series error when attempting to update a user profile in Iterable?